Add a doc about the Custom Javadoc Tags feature

[onewhl]

Description

Add a doc about the Custom Javadoc Tags feature.

Type of Change

• New documentation

Checklist (remove irrelevant options):

• New documentation is provided or existed one is altered
• All tests pass locally with my changes

[olganaumenko]

@onewhl, please find the screenshots below :)

[onewhl]

@korifey there are some screenshots and I'm not sure if it's ok to add them to repository. Where can we store them?

[amandelpie]

@korifey there are some screenshots and I'm not sure if it's ok to add them to repository. Where can we store them?

IMHO, we could a subfolder images in docs and them there if they are enough small

Or it should be located in the Readme of project or in Wiki

[onewhl]

@amandelpie each image is about 50Kb. It would be awesome to store images on some server and add links to the images to the doc. So, they would be rendered in markdown correctly and won't occupy space in the repository.

[amandelpie]

@korifey could please have a look and approve

[amandelpie]

@korifey please have a look, need to merge it