Doc: how to make changes to libasr

[certik]

Here is documentation how to make changes to libasr. This should be moved to README and our documentation.

To make changes to the libasr/src/libasr directory, simply make changes there and ensure things work locally. Then:

1. cd libasr
2. git checkout -b myfix
3. git add -u # and other files as needed
4. git commit
5. git push mylfortran myfix # Where mylfortran is your fork of LFortran, for example git@github.com:certik/lfortran.git.
6. Click on the link in the terminal to create a PR against LFortran
7. cd ..
8. git checkout -b myfix2
9. git add libasr # and other files from LPython as needed
10. git commit
11. git push mylpython myfix2

The step 9. updates libasr to a new hash, and the step 5. is what allows GitHub to see it, so that everything works at the CI.