Add documentation

Closes gh-3

Author: rainboyan

docs/guide/introduction.adoc

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+GORM Data Services also support returning RxJava 1.x https://reactivex.io/RxJava/1.x/javadoc/rx/Observable.html[rx.Observable] or https://reactivex.io/RxJava/1.x/javadoc/rx/Single.html[rx.Single] types.
+
+NOTE: RxJava 2.x support is planned for a future release

docs/guide/support.adoc

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+For support, please use the https://github.com/orgs/graceframework/discussions[Grace Discussions] or open an issue on link:{github}/issues[Github Issues].

docs/guide/toc.yml

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+introduction: Introduction
+usage: Usage
+support: Support

docs/guide/usage.adoc

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+To use the RxJava support you need to ensure that the `grace-datastore-gorm-rx` dependencies is on the classpath by adding the following to `build.gradle`:
+
+[source,groovy,subs="attributes"]
+.build.gradle
+----
+implementation "org.graceframework:grace-datastore-gorm-rx:{version}"
+----
+
+For example:
+
+[source,groovy]
+----
+import rx.*
+
+@Service(Book)
+interface BookService {
+   Single<Book> findOne(String title)
+}
+----
+
+When a `rx.Single` is used then a single result is returned. To query multiple results use an `rx.Observable` instead:
+
+[source,groovy]
+----
+import rx.*
+
+@Service(Book)
+interface BookService {
+   Observable<Book> findBooks(String title)
+}
+----
+
+For regular GORM entities, GORM will by default execute the persistence operation using RxJava's https://reactivex.io/RxJava/1.x/javadoc/rx/schedulers/Schedulers.html#io()[IO Scheduler].
+
+NOTE: For RxGORM entities where the underlying database supports non-blocking access the database driver will schedule the operation accordingly.
+
+You can run the operation on a different scheduler using the `RxSchedule` annotation:
+
+[source,groovy]
+----
+import rx.*
+import grails.gorm.rx.services.RxSchedule
+import grails.gorm.services.Service
+import rx.schedulers.Schedulers
+
+@Service(Book)
+interface BookService {
+
+   @RxSchedule(scheduler = { Schedulers.newThread() })
+   Observable<Book> findBooks(String title)
+}
+----