Update documentation for Equals()

This is for PR #190

index.md

diff --git a/index.md b/index.md
index 88ff564..6a4018a 100644
--- a/index.md
+++ b/index.md
@@ -321,6 +321,8 @@
 
 It is an error for both `ACTUAL` and `EXPECTED` to be nil, you should use `BeNil()` instead.
 
+When both `ACTUAL` and `EXPECTED` are a very long strings, it will attempt to pretty-print the diff and display exactly where they differ.
+
 > For asserting equality between numbers of different types, you'll want to use the [`BeNumerically()`](#benumericallycomparator-string-compareto-interface) matcher.
 
 #### BeEquivalentTo(expected interface{})