Add FSharp.Data Tests and Documentation to work with JSonValue

[jkone27]

Would be very useful to have unit tests (i see the project is empty) and/or documentation to work with FSharp.Data.JsonValue lenses

[wallymathieu]

The tests that execute are the same as can be found for other implementations, there are some ifdefs in the source: Note the references in fsproj here: https://github.com/fsprojects/Fleece/blob/v0.9.0/test/Tests.FSharpData/Tests.FSharpData.fsproj#L9-L10

The ifdefs are here: https://github.com/fsprojects/Fleece/blob/v0.9.0/test/Tests/Lenses.fs#L10-L15

Having additional tests and documentation would be neat though 😃