Introduce keepachangelog.com

[tormi]

Let's integrate https://keepachangelog.com/en/1.0.0/ into our flow. An example of a workflow from one of our projects:

## Release procedures

### Normal release

- Make sure `master` branch you're working on is up-to-date:

        git checkout master
        git pull

- Merge `master` branch into `production` to create a release:

        git checkout production
        git pull
        git merge master --no-ff --no-commit

- Before committing the merge change in CHANGELOG.md the title of [[Unreleased]](./CHANGELOG.md#Unreleased) section into the corresponding version numbered release title, e.g.:

>       ## [1.3.0] - 2020-11-17

- Add the changes in Changelog and commit merge:

        git add CHANGELOG.md
        git commit

- Tag your release on **`production`** branch, e.g.:

        git tag 1.3.0

- Push the changes to start deployment:

        git push
        git push --tags

- Back in `master` branch prepare the upcoming version for development phase and rebase to `production` ...

        git checkout master
        git rebase production --rebase-merges

- ... use `update-version.sh` script to increment version numbers (increment _minor_ version number by one) in `package.json` and `package-lock.json` files, e.g.:

        ./tools/update-version.sh 1.4.0
        git add package-lock.json
        git add package.json

- ... add new [Unreleased] title into CHANGELOG.md file
- ... and commit the result:

        git add CHANGELOG.md
        git commit -m "Init new development version"
        git push

### Hotfix release

- Make sure hotfix release is added to CHANGELOG.md, e.g.:

>       ## [1.3.1] - 2020-11-18
>
>       ### Fixed
>
>       - XXX-NN: Description of fix

- Also make sure version numbers in `package.json` and `package-lock.json` match with the hotfix version. You can use `update-version.sh` script to update them (increment _patch_ version number by one), e.g.:

        ./tools/update-version.sh 1.3.1

- Accept and merge the PR of hotfix branch / merge the hotfix branch into `production` branch:

        git checkout production
        git pull
        git merge hotfix/XXX-NN-your-hotfix-branch-title --no-ff

- Tag your release on **`production`** branch, e.g.:

        git tag 1.3.1

- Push the changes to start deployment (or just push tag if you merged a PR earlier):

        git push
        git push --tags

- Back in `master` branch (make sure it's up-to-date) rebase to `production` ...

        git checkout master
        git pull
        git rebase production --rebase-merges

- Resolve potential conflicts ...
  - Check history in CHANGELOG.md file
  - Conflicting version numbers in files should refer to version in `master` branch
- ... and (force) push:

        git push -f