Add documentation about generating exercise READMEs

[kytrinyx]

In the discussion about re-evaluating cross-track consistency (exercism/discussions#158) we concluded that we would make each exercise stand-alone, and stop generating READMEs on the fly.

In order to ease the burden of keeping READMEs up-to-date, we decided to add some tooling to make it easy.

I've reworked configlet to have subcommands. lint now does what the bare configlet command used to do, and generate generates the exercise READMEs. All of this new configlet code is still in pull requests:

• Add README configlet#23
• build: Support more platforms configlet#24

[kytrinyx]

I'm going to go ahead and merge this so that I can refer to the document in the pull requests that add the templates to all the tracks.

I imagine that the document will need improvement, so please do add comments and suggestions, and I'll make the improvements in subsequent pull requests.