Document processors to pass on all available metadata

[sdruskat]

The current processors (as of f02ff2a) don't process all metadata available in the harvesting contexts.

Examples for this are

• Git branch doesn't get passed to git harvesting context #120
• CFF process workflow doesn't record email addresses (and potentially other data) #112
• The fact that data from the CodeMeta harvest data isn't processed at all at the moment 😲.

This is of course bad 💀, and should be fixed for the existing processors.

Additionally though, and this is the scope of this issue, we shuold document how to extend hermes for a new datasource, and include the information that per default (unless configuration says differently), all metadata should be passed on to the CodeMetaContext (currently used for processing output).

[led02]

This issue seems a bit superseded. E.g., there is a CodeMeta harvester available…

However, we still need to collect everything but we should also explain, how and where to store the data.

See also #153