Add new hook for running terraform-docs which will replace content of README.md (using Python)

[RothAndrew]

Adds a new hook called terraform_docs_replace which uses the documentation found at the top of main.tf to recreate README.md. I'm open to suggestions on the name, I don't love it.

Fix #17

I know this is going to probably come across as "going against the grain" a little bit, but I wanted to write it because this is how I've done all my Terraform documentation, and this seemed like the right place for it to live.

It's not super thoroughly tested, but from the testing that I have done it looks very promising. Switching to Python instead of a shell script actually gives better OS compatibility because it works anywhere Python works vs the bash scripts that only work on OS's in the Unix family. Also, we know Python is already installed because you need Python to run pre-commit

[antonbabenko]

Thanks for the PR. It looks pretty good, but I propose to make few customization to make it more flexible:

1. Make filename README.md customizable via args.
2. Where main.tf comes from? Where it is used? Why main.tf?
3. Move setup.py and package files inside a folder (eg, terraform_docs_replace) to not mix root of the repo.
4. sed command can be different between systems. Since this is already in Python I would recommend to use native Python methods and replace sed.

[RothAndrew]

Make filename README.md customizable via args.

Sure that sounds reasonable.

Where main.tf comes from? Where it is used? Why main.tf?

main.tf is the standard entrypoint for a Terraform module. terraform-docs looks for documentation at the top of main.tf to add to the markdown document it creates. I'm not explicitly telling it to do that.

Move setup.py and package files inside a folder (eg, terraform_docs_replace) to not mix root of the repo.

According to the docs setup.py has to stay at the root in order to get picked up by pip. Note: a single setup.py file will support any and all future pre-commit hooks written in Python. All you have to do is add a line to the console_scripts array in entry_points.

sed command can be different between systems. Since this is already in Python I would recommend to use native Python methods and replace sed.

Makes sense

[antonbabenko]

Looks good. :)

Could you also add a note to README for developers (not users) something like "It is possible to support any and all future pre-commit hooks written in Python. All you have to do is add a line to the console_scripts array in entry_points."

[RothAndrew]

Don't merge this yet. I'm not done with it.

[RothAndrew]

Actually, maybe I am. @antonbabenko if you're fine with leaving the call to sed in for now this can be merged. I'll open another PR to change the sed call to native Python.

[RothAndrew]

Ready to merge. I'm done making changes unless there are more requested.

[antonbabenko]

Great! v1.8.0 has been released.

Thanks @RothAndrew for the work!

[robinbowes]

main.tf is the standard entrypoint for a Terraform module. terraform-docs looks for documentation at the top of main.tf to add to the markdown document it creates. I'm not explicitly telling it to do that.

This is not clear - could you update the docs to explain this, @RothAndrew ?