Document how to cleanup the bst cache: locally and in the server side

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/700)
In GitLab by [[Gitlab user @jjardon]](https://gitlab.com/jjardon) on Oct 9, 2018, 10:45

## Background
I can not dinf any documents that explain how to do this

If the only way for now is to `rm -rf` the folder, I think It still is valuable information to actually make this process clear

## Acceptance Criteria
- [ ] How to cleanup the cache locally is documented
- [ ] How to cleanup the cache in the server is documented

The issue names no repository files, tests, or entry points; start by locating the existing documentation for cache management and the local and server-side cache configuration. Document the cleanup procedure for both environments, including whether removing the cache directory is supported, and verify that both acceptance criteria are covered.

Written by the indexing model from the issue text.