docs: add a section on "Learning the codebase" to `CONTRIBUTING.md` (#351)
docs: add a section on "Learning the codebase" to `CONTRIBUTING.md` (#351)

- thought that as it's somewhat? fresh in my head (pending if you
  consider years of sporadic contributions), it would be really good to
  document for new contributors who don't know where to start

- other than being good practice, thought this could be useful for a
  number of reasons:
  1. helpful resources like the TSConfig Reference, the TS Wiki, and the
     Rollup Plugin docs, that I read through with some frequency
     - (and contribute lots to the TSConfig Reference too, it's by far
      the most useful resource in the ecosystem, IMO)
  2. explaining directly in the docs how sparsely documented the TS
     Compiler API is
     - like it really makes things difficult and every time I'm looking
       to contribute a bigger bugfix I look at the TS Wiki and don't
       get a super helpful answer there
  3. this codebase is actually fairly simple (it's the main reason I
    became a contributor, as I've stated in a few issues) and truly not
    that hard to get started with
    - but there are some rabbit holes you can go down that can scare
      away some contributors, like the cache code or the logging nuances
      - so this gives a bit of a guide to not fall into some rabbit
        holes and start off with the less complex bits of code
  4. encourage more contributors to make some PRs like I first did!
ezolenko/rollup-plugin-typescript2,ezolenko/rollup-plugin-typescript2