add info why it is better than https://github.com/brigade/overcommit

[matkoniecz]

It seems a very similar project and it is no obvious whetever one is better/for different purposes.

It would be nice to mention documentaton for what purposes this project is better

[asottile]

It was mentioned explicitly at some point but perhaps that has been removed from the docs.

For example:

• http://pre-commit.com/#advanced
• Doesn't require you to install a bunch of stuff to your system (zero setup other than having pre-commit installed)
• Easy to integrate any tool (for example, you could run overcommit as a pre-commit hook)

[asottile]

I'm going to complete this in our documentation at some point, for now we'll track that progress there: pre-commit/pre-commit.com#128