doc: port CHANGELOG.md to master
Fixes the CHANGELOG.md we link to from the website. I left out the version changes as suggested in #2448 and also removed the known issue about memory leaks as per the discussion over there.
Primarily looking for a LGTM here
obsoletes: https://github.com/nodejs/node/pull/2448 fixes: https://github.com/nodejs/node/issues/2456 fixes: https://github.com/nodejs/node/issues/2502
cc: @nodejs/collaborators @nodejs/release