readme: link to godoc for current main branch (#893)

This is a release-specific change corresponding to #886

Author: dylrich

Diff for: README.md

@@ -1,6 +1,6 @@
 git2go
 ======
-[![GoDoc](https://godoc.org/github.com/libgit2/git2go?status.svg)](http://godoc.org/github.com/libgit2/git2go) [![Build Status](https://travis-ci.org/libgit2/git2go.svg?branch=main)](https://travis-ci.org/libgit2/git2go)
+[![GoDoc](https://godoc.org/github.com/libgit2/git2go?status.svg)](http://godoc.org/github.com/libgit2/git2go/v31) [![Build Status](https://travis-ci.org/libgit2/git2go.svg?branch=main)](https://travis-ci.org/libgit2/git2go)
 
 Go bindings for [libgit2](http://libgit2.github.com/).