Maintenance Mode and syde/phpcs

[tfrommen]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes/features)
• Docs have been added/updated (for bug fixes/features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Inform users about shifting to maintenance mode, and the new syde/phpcs package.

What is the current behavior? (You can also link to an open issue here)

Users do not know about maintenance mode, nor syde/phpcs.

What is the new behavior (if this is a feature change)?

The README informs about the package being in maintenance mode, and recommends migrating to the new syde/phpcs package. Composer will suggest using the package, too.

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

Other information:

[tfrommen]

@Chrico @gmazzap @antonioeatgoat with the first stable release of syde/phpcs, I feel this is good to go now.

I am thinking this would be a patch-level release, 2.0.1, so many people will get this information (also the Composer-based suggestion) relatively soon. Do you agree?