Add documentation for adding a new field to an entity

Basics

[X] The PR is rebased with current master
[X] I added a line to changelog.md
[X] Details of what I changed are in the commit messages
[X] References to issues, e.g. close #X, are in the commit messages and changelog
[ ] The buildserver is happy

[ ] I fully described what my PR does in the documentation
[ ] I fixed all affected documentation
[ ] I fixed the introduction tour
[ ] I wrote migrations in a way that they are compatible with already present data
[ ] I fixed all affected decisions
[ ] I added automated tests or a manual test protocol
[ ] I added code comments, logging, and assertions as appropriate
[ ] I translated all strings visible to the user
[ ] I mentioned every code or binary not directly written or done by me in reuse syntax
[ ] I created left-over issues for things that are still to be done
[ ] Code is conforming to our Architecture
[ ] Code is conforming to our Guidelines
[ ] Code is consistent to our Design Decisions
[ ] Exceptions to any guidelines are documented