Add usage instructions to README.md

Author: sgarner

README.md

@@ -1,3 +1,151 @@
 # vue-apollo-smart-ops
 
-Create TypeScript-typed operation functions for your Vue Apollo queries and mutations.
+Creates TypeScript-typed operation functions for GraphQL queries and mutations compatible with
+[Vue Apollo](https://apollo.vuejs.org/).
+
+This library is intended to be used together with the
+[`typescript-vue-apollo-smart-ops` plugin](https://www.graphql-code-generator.com/docs/plugins/typescript-vue-apollo-smart-ops)
+for [GraphQL Code Generator](https://www.graphql-code-generator.com/), but it may also be useful standalone.
+
+## Installation
+
+```shell
+npm install --save vue-apollo-smart-ops
+```
+
+## Smart Query Usage
+
+### `createSmartQueryOptionsFunction<TResult, TVariables>(query: DocumentNode, onError?: ApolloOperationErrorHandlerFunction): VueApolloSmartQueryOptionsFunction`
+
+Returns a generated function which returns a [Vue Apollo Smart Query options object](https://apollo.vuejs.org/api/smart-query.html#options)
+for the given query when called.
+
+> ⚠️ Note: The returned function is not meant to execute the query itself. It is only used to configure a Vue Apollo
+> Smart Query. The responsibility for executing the query lies with Vue Apollo.
+
+The returned function accepts an options object as its first argument, allowing variables and other parameters to be
+customised in runtime usage. Compared with creating an options object directly, the advantage here is that the options
+accepted by the generated function are fully type-checked based on the query definition - and without needing to pass
+type arguments at every usage.
+
+Using the [`@graphql-codegen/typescript-vue-apollo-smart-ops` plugin](https://www.graphql-code-generator.com/docs/plugins/typescript-vue-apollo-smart-ops)
+you can automatically generate options functions for all of the query operations in your project.
+
+The following example manually creates an options function and assigns it to the variable `useTodoListQuery`:
+
+```typescript
+const useTodoListQuery = createSmartQueryOptionsFunction<TodosQuery, TodosQueryVariables>(gql`
+  query Todos($skip: Int, $limit: Int) {
+    todos(skip: $skip, limit: $limit) {
+      id
+      title
+    }
+  }
+`);
+```
+
+This function can subsequently be called inside the `apollo` options of a Vue component to configure a Smart Query. The
+following example adds a `todos` Smart Query to a component, in Vue class component style:
+
+```typescript
+@Component<TodoList>({
+  apollo: {
+    todos: useTodoListQuery<TodoList>({
+      skip() {
+        return this.foo === 'bar';
+      },
+      variables() {
+        return {
+          limit: 10,
+          skip: 0,
+        };
+      },
+    })
+  }
+})
+class TodoList extends Vue {
+  todos: Todo[] | null = null;
+
+  get foo(): string {
+    return 'bar';
+  }
+}
+```
+
+### `@SmartQuery(options: VueApolloSmartQueryOptions)`
+
+The `@SmartQuery()` decorator works with your generated options functions to enable the declaration of Smart Queries
+within the body of a Vue class component, instead of in the component options. This helps to keep the data property and
+its query options together in one place.
+
+The following example is equivalent to the previous example but using the decorated syntax style:
+
+```typescript
+@Component
+class TodoList extends Vue {
+  @SmartQuery(
+    useTodoListQuery<TodoList>({
+      skip() {
+        return this.foo === 'bar';
+      },
+      variables() {
+        return this.vars;
+      },
+    }),
+  )
+  todos: Todo[] | null = null;
+
+  get foo(): string {
+    return 'bar';
+  }
+}
+```
+
+## Mutations Usage
+
+### `createMutationFunction(mutation: DocumentNode, onError?: ): MutationOperationFunction`
+
+Returns a generated function which executes a mutation and returns the result when called.
+
+The returned function requires a Vue app instance as its first argument (from which the `$apollo` client will be
+accessed), and accepts an options object as its second argument, allowing variables and other parameters to be
+customised in runtime usage. Compared with executing a mutation using the Vue Apollo client directly, the advantage here
+is that the options accepted by the generated function are fully type-checked based on the operation definition - and
+without needing to pass type arguments at every usage.
+
+Using the [`@graphql-codegen/typescript-vue-apollo-smart-ops` plugin](https://www.graphql-code-generator.com/docs/plugins/typescript-vue-apollo-smart-ops)
+you can automatically generate mutation functions for all of the mutation operations in your project.
+
+The following example manually creates a mutation function and assigns it to the variable `todoCreateMutation`:
+
+```typescript
+const todoCreateMutation = createMutationFunction<TodoCreateMutation, TodoCreateMutationVariables>(gql`
+  mutation TodoCreate($input: TodoInput!) {
+    todoCreate(input: $input) {
+      todo {
+        id
+        title
+      }
+    }
+  }
+`);
+```
+
+The following example demonstrates how to call this mutation from a method on a component:
+
+```typescript
+@Component
+class TodoList extends Vue {
+  async onClickCreateTodoButton(): Promise<void> {
+    const result = await todoCreateMutation(this, {
+      variables: {
+        title: 'Bake a cake',
+      },
+    });
+    
+    if (!result.success) {
+      throw new Error(`Failed to create Todo!`);
+    }
+  }
+}
+```