Improve the GraalVM native-image documentation (#1168)

1. I was unable to see the documentatoin in the table of contents
2. GraalVM native-image plugin should be in the diagram
3. Added quickstart steps for GraalVM by reusing our Travis installer.

Author: ScalaWilliam

src/sphinx/formats/graalvm-native-image.rst

@@ -12,6 +12,17 @@ Requirements
 
 You must have ``native-image`` of GraalVM in your ``PATH``.
 
+Quick installation
+~~~~~~~~~~~~~~~~~~
+
+To get started quickly, eg make ``native-image`` available in your ``PATH``,
+you may reuse the script that is used for sbt-native-packager's continuous integration.
+To do so, run the following. It will install GraalVM 1.0.0-rc8.
+
+.. code-block:: bash
+
+  source <(curl -o - https://raw.githubusercontent.com/sbt/sbt-native-packager/6e1ee230350ce86c37b39c75f35718ac4a7f0a26/.travis/download-graalvm)
+
 Build
 -----
 

src/sphinx/formats/index.rst

@@ -10,12 +10,12 @@ existing functionality. Currently the autoplugin hierarchy looks like this ::
                   +
                   |
                   |
-    +-------+  Universal  +--------+-------------+
-    |             +                |             |
-    |             |                |             |
-    |             |                |             |
-    +             +                +             +
-  Docker    +-+ Linux +-+       Windows     JDKPackager
+    +-------+  Universal  +--------+-------------+----------------+
+    |             +                |             |                |
+    |             |                |             |                |
+    |             |                |             |                |
+    +             +                +             +                +
+  Docker    +-+ Linux +-+       Windows     JDKPackager   GraalVM native-image
             |           |
             |           |
             +           +
@@ -49,3 +49,4 @@ To learn more about a specific plugin, read the appropriate doc.
    docker.rst
    windows.rst
    jdkpackager.rst
+   graalvm-native-image.rst