Add coverage badge

[BenediktBurger]

Add a coverage badge to the readme file.

Note regarding implementation:

• That is the workflow of the coverage report: https://github.com/MishaKav/pytest-coverage-comment/blob/main/.github/workflows/live-test.yml
• https://github.com/marketplace/actions/dynamic-badges describes how to prepare everything for a dynamic badge (this is the dependency of pytest-coverage-comment): create a gist, make it public, create a secret for that gist, add it to the repository
• adjust the first script with the information of the second: gist id, file_name, secret name
• Important: You have to use the "githubusercontent" URL to work, see Domain is blocked [github.com] badges/shields#7435

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
pyleco
test.py	61	61	0%	25–128
pyleco/core
internal_protocols.py	12	12	0%	25–70
message.py	62	2	98%	121, 148
TOTAL	211	75	68%

Coverage Summary

Tests	Skipped	Failures	Errors	Time
84	0 💤	0 ❌	0 🔥	0.603s ⏱️

[BenediktBurger]

@bklebel @bilderbuchi

Would you mind reviewing this small PR, such that links to the working versions are in the readme of the main version?

[bklebel]

LGTM, I have added a miniscule change to the wording in the README.md, which is of no great consequence, use it or leave it at your own discretion :)

[BenediktBurger]

Thanks.