Update README.md with note about main branch

[hdamker]

What type of PR is this?

Add one of the following kinds:

• documentation
• subproject management

What this PR does / why we need it:

Update the README.md with the note about the main branch (frequent changes) and the hint to rely only on published releases. As requested in last QoD community call.

Which issue(s) this PR fixes:

n/a

Fixes #

Special notes for reviewers:

Sentence copied over from the CHANGELOG.md, just adapted the highlighting.

Changelog input

Additional documentation

[hdamker]

As discussed within last QoD call. @tlohmar please have a look if it is fulfilling your expectations.

[hdamker]

@tlohmar thanks - got the issue and committed your suggestions.

[hdamker]

Minor edit for readability ... ready to merge? @tlohmar @shilpa-padgaonkar

[tlohmar]

Minor edit for readability ... ready to merge? @tlohmar @shilpa-padgaonkar

Looks good.