Missing installation instructions

[amossc]

Before submitting an issue, please make sure:

• [X] You're running Go 1.6 or later
• [ ] You've tried installing with go get -u to update dependencies

If you see the following error, you need to update to Go 1.6+:

$ go get github.com/zmb3/gogetdoc
# github.com/zmb3/gogetdoc
./ident.go:142: c.Val().ExactString undefined (type constant.Value has no field or method ExactString)

It would be useful to have the installation instructions in the README.md, not just the issue template.