Link to other resources instead of inlining their information

Link to other resources instead of inlining their information

The other places are more accurate and up-to-date.

- Link to `std-dev-guide` in CONTRIBUTING.md

  Thom and Mara said the guide is in reasonably good shape, and it's tailored more closely to people working on the standard library.

- Link to CONTRIBUTING.md instead of rustc-dev-guide in the main readme

  CONTRIBUTING.md has more information and also links the std-dev-guide.

- Link to forge for the list of tested platforms; the one in the readme
  was hopelessly out of date.