add a contributing guide

Questions about how to contribute come up from time to time, eg:
https://github.com/scottlamb/moonfire-nvr/issues/84#issuecomment-656065208
https://github.com/scottlamb/moonfire-nvr/issues/68#issuecomment-688938108

I hope this helps answer them. I think github adds a couple links to
a file called CONTRIBUTING or CONTRIBUTING.md so this filename gives it
some extra visibility.

design/api.md

@@ -25,13 +25,13 @@ Status: **current**.
 ## Objective
 
 Allow a JavaScript-based web interface to list cameras and view recordings.
+Support external analytics.
 
 In the future, this is likely to be expanded:
 
 *   configuration support
 *   commandline tool over a UNIX-domain socket
     (at least for bootstrapping web authentication)
-*   mobile interface
 
 ## Detailed design