Review README.md and CONTRIBUTING.md files

[jasonblais]

Review README.md and CONTRIBUTING.md files

Creating an issue for @jasonblais to review the two files and update where needed

1. README should really be for developers looking to contribute, and install guides, feature lists, etc.
2. contribution guidelines way out of date and missing a note about testers. Could base it off of platform contribution guidelines: Update contributing guidelines for platform repo mattermost#5455

[jasonblais]

Sorry @yuya-oc, I unfortunately won't have time to finish this up for end of this week..

I've started doing some research though, and added a few references below.

mattermost/mattermost#5455
https://github.com/mattermost/desktop/pull/350/files
https://github.com/nodejs/node
https://github.com/nodejs/node/blob/master/COLLABORATOR_GUIDE.md#landing-pull-requests
https://github.com/EionRobb/purple-mattermost

[yuya-oc]

As mentioned at #637 (comment), we should also consider to define how changelog should be written. Currently "issues" are listed trailing to the sentence, but "pull requests" are not.

[philip-peterson]

Just for cross-referencers' sake, I was also suggesting back there that some distinction be made between PR number indicators and Issue number indicators (perhaps PR #4 and Issue #4 as a convention). Right now when you see #34 in the changelog it could mean either PR #34 or Issue #34.

[amyblais]

Jira ticket: https://mattermost.atlassian.net/browse/MM-14062.