Document the usage of logical operators

[Obirah]

I had to implement a vertex query containing an or-Condition and just from reading the documentation it was not obvious for me how the logical operators .or(...) and .and(...) work. In order to find out I went through the LogicalSpec:

https://github.com/mpollmeier/gremlin-scala/blob/f64664c4d5b1731bf92045252b1290dd218b748a/gremlin-scala/src/test/scala/gremlin/scala/LogicalSpec.scala

It would be nice to have an overview of the operator usage in README.md

[mpollmeier]

agreed, and PRs are always welcome :)