Update versions supported to include go 1.16

Rather than directly adding `1.16`, I thought better to reword to state the minimum version supported... this way the line doesn't have to get updated with every new release, but instead only when dropping releases.

README.md

@@ -323,7 +323,7 @@ To update Testify to the latest version, use `go get -u github.com/stretchr/test
 Supported go versions
 ==================
 
-We support the three major Go versions, which are 1.13, 1.14 and 1.15 at the moment.
+We currently support the most recent major Go versions from 1.13 onward.
 
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