List available methods in README.md (#22)

Author: dstj

README.md

@@ -4,6 +4,20 @@
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+### Available extension methods
+
+- `BeEquivalentTo()`
+- `ContainSingleElement()`
+- `ContainSubtree()`
+- `HaveCount()`
+- `HaveElement()`
+- `HaveValue()`
+- `NotBeEquivalentTo()`
+- `NotHaveElement()`
+- `NotHaveValue()`
+
+See "in-code" description for more information.
+
 ### Usage
 
 Be sure to include `using FluentAssertions.Json;` otherwise false positives may occur.