Add changesets to ease versioning

[ukorvl]

We use changesets in niljs.
Actually it works quite well and fixes lots of common problems related to versioning npm package.

Typical problems on automated versioning:

• It is complicated to agree with all developers when to patch version, when to use minor or major.
• When both people want to upgrade version simultaneously it will be a mess and merge conflicts in package.json
• Keeping several versions actual at the same time. It means not to just bump version all the time, but have different branches with different versions and update all of them.
• How to mention in changelog and release all the changes and determine new version basing on all of the changes since past version? There can be minor, patch and major updates.

We can take changesets from niljs as an example and add it here to have clear changelog and stable process of releasing new versions.