Make a guide to contributing to this docs repo

[Mr0grog]

Explain Hugo, how to build, how some docs come from other repos, so pull requests should be there instead of here, etc.

[Mr0grog]

Should also talk about what various types of docs should be/should cover. I’ve added this to a few issues about concept docs, but it should really be in this guide:

Concept docs are concise, high-level descriptions of various core concepts in IPFS. They live in the content/guides/concepts folder and should strive to answer:

1. What is this?
2. How does it relate to the rest of IPFS?
3. How can (or should?) you use it? (Note: sometimes the answer is that you should understand it, but not use it. Bitswap is probably a good example.)
4. Where do you go to learn more?
5. What is the current state of affairs?