Update CHANGELOG.md
Update CHANGELOG.md

Adds in high level description of the functionality provided by the 2.0.0 release

slackapi/node-slack-sdk,slackapi/node-slack-sdk,slackhq/node-slack-client,slackhq/node-slack-sdk,slackapi/node-slack-sdk,peernohell/node-slack-client,slackapi/node-slack-sdk,peernohell/node-slack-client