Document configurable entity ignoring (#511)

[borekb]

Document versionpress/versionpress#511. Things worth mentioning:

• Format used for ignoring
• What is ignored by default
• What cannot be ignored using the current format
• Link to query language - should be a separate docs issue

[JanVoracek]

We don't want this to be part of the documentation yet.

1. The documentation is for common users and the configuration is not user friendly.
2. There is the schema-readme.md for tech users.

[borekb]

Moving to 4.0.