docs: first commit in order to improve doc (JavaDoc, GH)

[The-Lum]

Hi PlantUML team, and all,

Here is a Pull Request, with first commit to attempt to answer to:

• [Wanted feature][docs] Improve Doc (JavaDoc, GH) [package-info.java, readme.md] #1547

[Sorry for the alone commit a40c98b (I clicked too quickly), and it will be necessary to change the filename on lowercase... due to Java Project, and to go near package-info.java]

Then a template for all readme.md will be perhaps necessary (here is on this PR a proposal):

# Directory Documentation for `<<directory_name>>`

## Description
This package provides classes used to manage ...

## Link

## Reference

## Credit

## Misc.

🛠 TBC... (And open to discussion and debate)
Thanks for all new contributions...

Regards,
Th.