Document the current capabilities and limitations of pre-commit hooks

[grempe]

Referencing pull request: #235 (comment)

There is a lack of documentation regarding what pre-commit hooks (and probably others as well) can do as a result of implementation details of overcommit.

For example, pre-commit hooks implemented in overcommit cannot have side effects. You cannot modify, add, remove files from within a pre-commit hook since the workflow for running that hook involves stashing all unindexed content in the work dir, doing a git reset --hard, and stash apply. These actions overwrite any changes that a script might have made.

Currently this behavior is undocumented.

[grempe]

I referenced the wrong comment. It should be: #235 (comment)

[sds]

Added some documentation in 6c88caf.