development documentation

[jodygarnett]

Development documentation is currently split between codebase, wiki and the user manual.

I would like to resolve this according to direction from the core-geonetwork README:

Developer documentation is also in the docs submodule but is being migrated out of that project into the Readme files in each module in the project. General documentation for the project as a whole is in this Readme and module specific documentation can be found in
each module (assuming there is module specific documentation required).

Bringing all the developer docs into a series of README.md files is too much for one commit, here are the first couple changes:

• updates build instructions for IntilliJ
• Offers a few more creature comforts like markdown CONTRIBUTING.md and LICENSE.md files
• copy over build from source code instructions

This PR allows draft edits by maintainers so please just edit the pages directly in GitHub (rather than cycle through the review process).

Notes:

• Eventually we need updates build instructions for maven
• Use of add-schema.sh is documented on Use unpack to support use of schema plugins as artifacts #4701
• Build instructions for eclipse should be updated
• Move dev doc to the doc instead of wiki. doc#113

[jodygarnett]

I was reminded of this PR checking out 3.10.x and finding outdated instructions.

[jodygarnett]

@juanluisrp are you in position to review?

[jodygarnett]

I have squashed history making this easier to apply