Specs for Continous Integration and Code Reviews

[kba]

trafficstars

We briefly talked about those in the Tech Call today and decided to make these part of the spec, hence this PR.

I took the liberty of updating the list of CI providers (we do not use Travis CI anymore, but we do use Github Actions), otherwise they are unchanged from @mweidling's inital drafts.

[kba]

See also: https://hackmd.io/g-8QuOAnSQeqnb8PRNjFbQ (ht @bertsky)

• https://github.com/OCR-D/ocrd-website/blob/master/site/en/developer-guide.md (not linked into ocr-d.de)
• https://github.com/OCR-D/ocrd-website/blob/master/site/en/dev-best-practice.md (linked into ocr-d.de)
• https://github.com/OCR-D/ocrd-website/wiki/OCR-D-Code-Review-Guidelines (new, just on the Wiki)

[stweil]

Meanwhile this PR is older than a year. Is there any chance that it will be merged (with my suggestions)?