Warn of deprecation

[MatteoVH]

It seems as though this service has been eschewed in favor of wdio-visual-regression-service considering that:

• The last commit was almost a year ago
• The official docs for v4+ list wdio-visual-regression-service and don't mention this repo
• Talks about joining forces with wdio-visual-regression-service have stalled for almost a year
• The official support of WebdriverIO is 2 major versions behind
• Dependencies are being deprecated/falling out of date

Please put a clear deprecation warning up at the top of the README if development has stopped.

[fatso83]

This has already happened three weeks ago.

[wereHamster]

The readme should ideally point to wdio-visual-regression-service as a preferred replacement.