Created new HOWTO section 'Implementing adapters'.

Section includes notes about
-   Implementing `OptiqPrepare.Context`
-   Getting enumerable from `OptiqPrepareImpl`
-   `JavaTypeFactory` and `Type` instances (referring to https://github.com/julianhyde/optiq/issues/54)

There are some overlapping content and TODO sections in https://github.com/julianhyde/optiq-csv/blob/master/TUTORIAL.md

Maybe there could be a separate page dedicated to implementing custom adapters on optiq side. optiq-csv would be just an example project then.