Improve pull_request_template and add "inline documentation" conventions to README

[ttarczynski]

It's related to: #58

Changes/added features:

• Add a successful ./run.sh to PR checklist (in .github/pull_request_template.md)
• Add "inline documentation" conventions (to README.md)

The following short checklist should be used to make sure your PR is of good quality, and can be merged easily:

• [x] follows project conventions
• [x] follows project principles
• [x] CI checks pass
• [x] mergeable to main
• [x] resolved issues linked

Reviewers: @ianmiell @ttarczynski @sanyer