Add documentation about JPMS (#758)

* Add documentation about JPMS

* Format

Author: nakamura-to

docs/index.rst

@@ -59,6 +59,7 @@ User Documentation
    lombok-support
    kotlin-support
    slf4j-support
+   jpms-support
 
 About Doma
 ==========

docs/jpms-support.rst

@@ -0,0 +1,52 @@
+=============
+JPMS support
+=============
+
+.. contents::
+   :depth: 3
+
+Overview
+========
+
+Doma supports the Java Platform Module System (JPMS) introduced with Java 9.
+
+Modules
+=======
+
+Doma provides the following modules:
+
++----------------+------------------------------------+
+| Artifact ID    |  Module Name                       |
++================+====================================+
+| doma-core      |  org.seasar.doma.core              |
++----------------+------------------------------------+
+| doma-kotlin    |  org.seasar.doma.kotlin            |
++----------------+------------------------------------+
+| doma-processor |  org.seasar.doma.processor         |
++----------------+------------------------------------+
+| doma-slf4j     |  org.seasar.doma.slf4j             |
++----------------+------------------------------------+
+
+Usage
+=====
+
+Doma uses reflection to access Entity properties.
+For this to work, you have to open packages containing Entity classes:
+
+.. code-block:: java
+
+    module example.app {
+      requires org.seasar.doma.core;
+      requires org.seasar.doma.slf4j;
+
+      opens example.app.entity;
+    }
+
+You can open your module instead of packages:
+
+.. code-block:: java
+
+    open module example.app {
+      requires org.seasar.doma.core;
+      requires org.seasar.doma.slf4j;
+    }