Harmonise requirements towards markdown formatting

[ekatef]

Currently CI fails for every PR which introduces any changes into README. The reason in that requirements of github linter used do not quite match with README formatting. In particular, there are following points to be fixed:

• line length is too big (100..250 vs 80 default);
• an exclamation mark in a header.

Possible solution:

1. Include markdown linter into pre-commit
2. Change linter defaults

[ekatef]

A list of markdown linters can be extracted from a pre-commit list.

Defaults of Github Markdown linter are explained here.

An example of tackling a similar issue https://github.com/IBM/ibmi-oss-docs/pull/53