[W.I.P.] add postBuild, binder badge, and a bit more in README

[gaelforget]

With included changes one can try things out on mybinder.org, for example @

https://mybinder.org/v2/gh/gaelforget/Ocean-Plastic-Assimilator/gfdev01?urlpath=lab

But aside from changes included in this PR, I get an error show below (both on cloud instance and local machine).

Would it be easy to fix (either before merging this PR or afterwards) ?

# To activate this environment, use
#
#     $ conda activate ocean-plastic-assimilator
#
# To deactivate an active environment, use
#
#     $ conda deactivate

jovyan@jupyter-gaelforget-2docea-2dtic-2dassimilator-2dn05689s9:~$ conda activate ocean-plastic-assimilator
(ocean-plastic-assimilator) jovyan@jupyter-gaelforget-2docea-2dtic-2dassimilator-2dn05689s9:~$ python examples/assimilate_double_gyre.py
Traceback (most recent call last):
  File "examples/assimilate_double_gyre.py", line 1, in <module>
    from src.run_assimilator import run_assimilator
ModuleNotFoundError: No module named 'src'

with this python version being used

Python 3.8.8 | packaged by conda-forge | (default, Feb 20 2021, 16:22:27) 
[GCC 9.3.0] on linux

[gaelforget]

note : binder link in README.md should be changed once merged / before merging to point to https://github.com/TheOceanCleanupAlgorithms/Ocean-Plastic-Assimilator (instead of my fork currently in PR)