chore: adding go.mod

Update VERSION_HISTORY.md and MIGRATION_GUIDE.md (golang-jwt#27)

* Update VERSION_HISTORY.md
* Updated README and migration
* Added replacement command

documentation around expected key types

3.1.0 changelog

Merge pull request golang-jwt#77 from dgrijalva/release_3_0_0

Release 3.0.0

release notes

version history update

version history

Merge pull request golang-jwt#74 from patbaker82/ecdsa

Add ECDSA signatures