Versioning, Change log and release instructions

[rap1ds]

This PR defines a set of practises for keeping versioning/changelog/upgrade notes up-to-date:

Todo

• Add CHANGELOG.md. Follow the best-practices from keepchangelog.com
• Add UPGRADE.md. This file contains upgrade notes
• Add RELEASE.md. This file contains steps how to make a new release
• Modify CONTRIBUTING.md: Add a new bullet about updating CHANGELOG
• Upgrade README: Add links to the new files. Say explicitely that Sharetribe is using Semantic Versioning.

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[rap1ds]

I'm still wondering one thing: I think we don't need to do a new release each time we merge a Pull Request. We can use common sense to gather a nice group of additions/fixes and then make a new release. Which may lead to a problem: If I want to make a new release to get my PATCH level change released, how do I know if there are unreleased MAJOR or MINOR level changes? Should we explicitly add that information to the CHANGELOG so that by reading the CHANGELOG you know what part of the version number to increase?

Here's an example:

CHANGELOG.md

## [Unreleased]

### Added
- MINOR Added support for payment gateway xyz [#99999](https://github.com/sharetribe/sharetribe/pull/99999)

### Fixes
- PATCH Fixes typos in README.md
- PATCH Fixed broken link in About page

By looking at this CHANGELOG it's obvious that the next version number needs to increase the MINOR version.

[bladealslayer]

I think it would be nice if every PR comes with a corresponding entry in CHANGELOG.

[kpuputti]

I know this isn't legally binding text, but should this be stated with a warning? Something like "This project adheres to Semantic Versioning where possible." etc. Otherwise we might have people being surprised when a minor version change breaks existing functionality. We cannot really account for all cases like a library with a public API.

[rap1ds]

That's very true. I'll change this as you suggested.

[rap1ds]

@bladealslayer I totally agree. I added a note about that in the CONTRIBUTING.md https://github.com/sharetribe/sharetribe/pull/1658/files#diff-6a3371457528722a734f3c51d9238c13R15