Update and enhance README file(s)

[bockthom]

(1) I just noticed that our README file still points to the already archived version of siemens/codeface.
I suggest to replace the outdated pointer by a link to se-sic/codeface (but adding a comment that codeface was originally developed by Siemens).

(2) In addition, it might be useful to state which data of the main data sources (commits, mails, issues) can be extracted by which tool (e.g., by adding a link to the respective tool, such as codeface or GitHubWrapper). However, I am not sure where to add this information. As coronet relies on codeface-extraction instead of directly using the outcomes of the different tools, maybe the README of codeface-extraction would be more suited to that, and we can just point to the codeface-extraction in the README of coronet for further reading upon how to extract data from the different main data sources.

@hechtlC: What do you think?

I would like to fix part (1) ASAP; part (2) is not as urgent.