Add a Contributing section to the docs, and cite it in error message

[sideshowbarker]

This change adds a Contributing section to the README.md, and cites it from the error message we emit when a string is passed to Rope≫Append() that’s larger than the allowed string size. Relates to #153, as an alternative to #157.

[domenic]

This is super-great. I have some editorial nits I'd be happy to correct myself (e.g. use more code font), but at a high-level my only question is:

• Should we try to document AsString vs. ExtractAll() vs. ... any other relevant patterns? That's what took me a long time to figure out when doing 3b11b2d . I just assumed "oh, strings are good, let me use AsString" and that turned out to be a terrible idea.

• Should we be more explicit about how to convert between ropes and cutropes? (It seems like the general pattern is, use Scratch?)

Your code examples use ExtractAll(), but say "happens to be a cutrope". I think what I found over the course of recent work is, some parts of Wattsi really really expect ropes, not strings, so in general you should be paying a lot of attention to input types, and know clearly how to convert between cutropes (a common return value) and ropes (a common argument type).