.pre-commit-config README example not recommended by pre-commit

[bk-equityzen]

The readme recommends to use the following config:

repos:
-   repo: https://github.com/ambv/black
    rev: stable
    hooks:
    - id: black
      language_version: python3.6

But the pre-commit docs don't recommend this. This causes different developers getting different versions of black depending on when they install pre-commit

One example of how this broke things is the recent change in the pyproject.toml target-version option from 'cpy27' to ['py27'] which broke for developers that installed the pre-commit hook later.

From https://pre-commit.com/#using-the-latest-version-for-a-repository:

pre-commit assumes that the value of rev is an immutable ref (such as a tag or SHA) and will cache based on that. Using a branch name (or HEAD) for the value of rev is not supported and will only represent the state of that mutable ref at the time of hook installation (and will NOT update automatically).

[asottile]

Duplicate of #420

[JelleZijlstra]

Closed as a duplicate.