* Added items to changelog for 2.0 release
* Removed unnecessary uses of std in doctests
* Gated `Context` behind `digest`
* Fixed noncompiling doctest when only `digest` is enabled
* README feature flag list mostly done
* Copied changelog to readme
* Redid the malleability section in README
* Added CONTRIBUTING.md
* Bumped version number to 2.0.0-pre.0; small changes to README
* Updated changelog for #277
* Added pem feature description
Co-authored-by: pinkforest(she/her) <36498018+pinkforest@users.noreply.github.com>
