Update Readme check to include missing plugin header

[ernilambar]

Related: https://github.com/WordPress/plugin-check/issues/198

In current readme validator, warnings are raised if following fields are missing:

• Tested up to
• Stable tag
• Contributors

We could also implement here in trunk a similar parity with the current readme validator.

Ref:

[ernilambar]

• Stable Tag - Since we already have special method for Stable Tag checking, I have added no_stable_tag in that same function. Related PR - https://github.com/WordPress/plugin-check/pull/403

[swissspidy]

414 seems fine at first glance, but I recommend asking the plugin review team whether it's useful for them

[ernilambar]

@foosantos We have already implemented missing Stable Tag. Would it be helpful if we also implement missing Tested up to and Contributors? By default contributor_ignored warning is ignored, so missing Contributors won't be warned.

[foosantos]

cc @bordoni

[davidperezgar]

Yes, we have to check that the plugin has a correct readme.

[ernilambar]

Fixed in https://github.com/WordPress/plugin-check/pull/414