Usage: add a section about @supports conditions

Author: carlosame

usage.html

@@ -1008,6 +1008,44 @@ <h3 class="subtema" id="media-queries">Media queries</h3>
 // would be the same one
 </code></pre>
 </div>
+<div class="tema" id="supports">
+<h2><code>@supports</code> rules</h2>
+<p>It is possible to use the object model to access the <code>@supports</code> conditions, which work in a similar way to Media Queries (but with <a class="codeitem"
+href="api/latest/io.sf.carte.css4j/io/sf/carte/doc/style/css/nsac/DeclarationCondition.html">DeclarationCondition</a>
+instead of <a class="codeitem" href="api/latest/io.sf.carte.css4j/io/sf/carte/doc/style/css/MediaQueryPredicate.html">MediaQueryPredicate</a>
+objects as predicates.</p>
+<p>For example:</p>
+<pre class="code"><code class="language-java">// Instantiate a css factory (we could also obtain it from a
+// pre-existing css4j object)
+AbstractCSSStyleSheetFactory cssFactory = new CSSDOMImplementation();
+
+// Create a style sheet
+AbstractCSSStyleSheet sheet = cssFactory.createStyleSheet(null, null);
+// Create a @supports rule
+SupportsRule supports = sheet.createSupportsRule("(display: flex)");
+
+// Get the condition
+BooleanCondition cond = supports.getCondition();
+
+// Get the condition type
+BooleanCondition.Type condType = cond.getType();
+// It is a BooleanCondition.Type.PREDICATE
+
+// So we cast it to DeclarationCondition
+DeclarationCondition dCond = (DeclarationCondition) cond;
+String predName = dCond.getName();
+// It is "display"
+
+// Now obtain the value
+LexicalUnit value = dCond.getValue();
+LexicalUnit.LexicalType luType = value.getLexicalUnitType();
+// It is a IDENT
+
+// Get the string value
+String condValue = value.getStringValue();
+// It is "flex"
+</code></pre>
+</div>
 <div class="tema" id="sheetsets">
 <h2>Style sheet sets</h2>
 <p>The library supports alternative style sheets: see CSSDocument's methods