Update README.md

Author: datsom1

README.md

@@ -127,3 +127,22 @@ You can also import the Python library into your own code like so:
 `diff` will now contain the same data structure as the output in the `--json` example above.
 
 If the columns in the CSV have changed, those added or removed columns will be ignored when calculating changes made to specific rows.
+
+## Full CLI Helptext
+
+`Usage: db-diff \[OPTIONS\] PREVIOUS CURRENT
+
+  Compare the differences between two CSV or JSON files to find differences.
+
+Options:
+  --format TEXT      Explicitly specify input format. Available (csv|tsv|json) \[default: auto-detect based on file extension\]
+  --encoding TEXT    Input File Encoding. Available: (utf-8|utf-16|utf-16le|utf-16be|latin1|cp1252|ascii|...) \[default: utf-8\]
+  --key TEXT         Column to use as a unique ID for each row (ex: --key=Id) \[default: first column if not specified\] 
+  --output TEXT      Output format. Available: (readable|json|jsonfile)  \[default: readable\]
+  --outputfile FILE  File to write JSON output to (only used with --output=jsonfile)
+  --showunchanged    If a record is changed, show ALL fields, not just the changed fields.
+  --time             Measure and display elapsed time for the diff operation
+  --version          Show the version and exit.
+  -h, --help         Show this message and exit.
+
+  Example: db-diff old.csv new.csv --key=Id --output=jsonfile --outputfile=diff.json`