* 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>