Improve README.md to indicate that "using FluentAssertions.Json;" is required (#20)

Author: dstj

README.md

@@ -4,4 +4,18 @@
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+### Usage
+
+Be sure to include `using FluentAssertions.Json;` otherwise false positives may occur.
+
+```c#
+using FluentAssertions;
+using FluentAssertions.Json;
+using Newtonsoft.Json.Linq;
+
+... 
+var actual = JToken.Parse(@"{ ""key1"" : ""value"" }");
+var expected = JToken.Parse(@"{ ""key2"" : ""value"" }");
+actual.Should().BeEquivalentTo(expected);
+```