Added how to create PR to HowToContribute.md

[l3str4nge]

A description has been added on how to create a new pull request. It should be useful for inexperienced programmers.

[l3str4nge]

Tests failed because we need to merge #79 or #76 because of:

  File "<frozen importlib._bootstrap_external>", line 728, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/home/runner/work/BookOmatic/BookOmatic/BookOmatic/urls.py", line 29, in <module>
    path('accounts/', include('django.contrib.auth.urls')),
NameError: name 'path' is not defined