Problem: the contributing docs are incomplete

[codegeschrei]

Solution: add missing parts and restructure reasonably

Check out the Write Code Guide for the BigchainDB repository.

• [ ] Replace Fix Bugs and Implement Features Section with something like Know What You Want to Write Code to Do
• [ ] Merge the PR examples of the Installation Guide for Developers with Pull Request Guidelines
• [ ] Update Pull Request Guidelines if needed (I bet there are some new structures, check

more inspiration for changes here but please check for their correctness in the driver before writing them down!

Open for comments/discussion/better ideas