We need to document LiteX CSRs!

[gsomlo]

I'd like to add a doc/CSR.md file to LiteX, documenting how CSRs work from both the hardware and software (programmer's API) perspective. I'm going to add a new comment below with a .md-ish formatted document, which we can collectively work on until it's more or less ready, after which I'll post a PR to add it to LiteX. If there's a better way to edit a doc before including it under ./doc, please let me know and I can start doing that (I'm a bit awkward with github, having been used to linux's mailing list based model of development and feedback solicitation, so apologies for any clunkiness :).
Comments, feedback, and direct edits of the following comment (if github allows it) much appreciated!

[gsomlo]

moved to https://github.com/enjoy-digital/litex/wiki/CSR-Bus -- please check there for the actual content of the documentation (gls -- 03/04/2020)

[mithro]

@xobs this is probably of interest to you.

[BracketMaster]

Nice!

[enjoy-digital]

@gsomlo: sorry for the delay and thanks for the documentation. I'll try in the next week to create a proper wiki for the project and just prepared things with topics i'd like to document. I just put your initial description here: https://github.com/enjoy-digital/litex/wiki/CSR-Bus. Feel free to edit it and update, i'll also try to work on the FIXME. We can keep this issue for discussing things until we are happy with the CSR documentation.

[BracketMaster]

@gsomlo I also have a simple working CSR example for ECP5 here if you'd like to reference that.