Create documentation for Systematic Literature Review (SLR)

[ThiloteE]

#333 attempted to add some documentation, but never was merged, because the info is outdated.

As it stands right now, people would need to experiment with the feature or to actually look into the code.

[koppor]

Blocked by https://github.com/koppor/jabref/pull/559.

We first need to finish the (new) SLR implementation and then work on the documentation.