chore: link to contributing guidelines from README.md

[tcrasset]

I feel it's helpful to show a newcomer how to start contributing, so I added the most basic steps. I'm leaving this intentionally very sparse so that you can add whatever you want @koxudaxi

[koxudaxi]

@tcrasset
Thank you for creating the PR.
We have the page for the contributing guide.
https://github.com/koxudaxi/datamodel-code-generator/blob/master/docs/development-contributing.md

https://koxudaxi.github.io/datamodel-code-generator/development-contributing/

But, the page is not linked from README.md.
This may be not very clear.
What do you think?

[tcrasset]

Ah, indeed I missed this one, probably due to the reasons you mention. It rather common to have a CONTRIBUTING.md in open-source projects, but I agree that we do not want to duplicate the information that is already in docs/.

Adding it to README.md seems like a fair compromise.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (e9b6edf) 100.00% compared to head (036d88d) 100.00%.
Report is 718 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##            master     #1561     +/-   ##
===========================================
  Coverage   100.00%   100.00%             
===========================================
  Files           11        33     +22     
  Lines         1020      3621   +2601     
  Branches       201       841    +640     
===========================================
+ Hits          1020      3621   +2601     

Flag	Coverage Δ
unittests	99.64% <99.59%> (?)

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Files Changed	Coverage Δ
datamodel_code_generator/__init__.py	100.00% <100.00%> (ø)
datamodel_code_generator/__main__.py	100.00% <100.00%> (ø)
datamodel_code_generator/arguments.py	100.00% <100.00%> (ø)
datamodel_code_generator/format.py	100.00% <100.00%> (ø)
datamodel_code_generator/http.py	100.00% <100.00%> (ø)
datamodel_code_generator/imports.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/__init__.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/base.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/dataclass.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/enum.py	100.00% <100.00%> (ø)
... and 23 more

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[tcrasset]

The old installer is deprecated