Improvements in readme

[AdrienTaylor]

Remarks from @Dralliag

The example from https://pepit.readthedocs.io/en/0.3.2/quickstart.html should include a few additional definitions (allowing to run the example)

• "fs" is undefined,
• "L" is undefined.

[AdrienTaylor]

Similarly:

• add links to the appropriate files in the "quickstart"
• a few sentences are difficult to parse in the "quickstart"