Document supported versions, upgrading, and version-specific PRs (#47)

Signed-off-by: Scott Rigby <[email protected]>

Author: scottrigby

README.md

@@ -146,3 +146,13 @@ Create and push a tag.
     -t, --tag         The name of the tag to create
     -s, --skip-push   Skip pushing the tag
 ```
+
+## Supported versions
+
+The previous MAJOR version will be supported for three months after each new MAJOR release.
+
+Within this support window, pull requests for the previous MAJOR version should be made against the previous release branch. For example, if the current MAJOR version is `v2`, the pull request base branch should be `release-v1`.
+
+## Upgrading
+
+When upgrading from `< v2.0.0` you will also need to change the usage in your scripts. This is because, while the [v2.0.0](https://github.com/helm/chart-testing/releases/tag/v2.0.0) release has parity with `v1`, it was refactored from a bash library to Go so there are minor syntax differences. Compare [v1 usage](https://github.com/helm/chart-testing/tree/release-v1#usage) with this (`v2`) version's README [usage](#usage) section above.