clarify usage of conventional-version and conventional-release

[olekhal]

The current examples for using conventional-version and conventional-release will no longer work. This is caused by a change in actions/checkout@v2 which no longer fetches tags by default. In v2 the default changed to only fetch the last commit.

to get the same behavior with v2 as in v1 it is necessary to use the fetch-depth:0 option

- uses: actions/checkout@v2
  with:
    fetch-depth: 0

[sasjo]

I'm aware of this and I think we should update all examples to use checkout@v2 in the right way.
The checkout@v2 action also renders our setup-git action moot as it will configure git CLI with the token credentials so it can be used in scripts.

[nikolay-bocharov]

The documentation for conventional-release is still stale