Add more info about reusable parts. (#527)

Add link to the parts wiki in serveral places and add a section to
the parts documentation explaining how to reuse parts.

LP: #1582499

docs/snapcraft-advanced-features.md

@@ -160,9 +160,9 @@ after:
 ```
 
 As we never define the `curl` part in the above example, `snapcraft` will
-check the Ubuntu Wiki, which is where we currently host examples of
-successful snapcraft parts. The build order in this case would be `curl`,
-then `main`.
+check the Ubuntu [Wiki][wiki],
+which is where we currently host examples of successful snapcraft parts.
+The build order in this case would be `curl`, then `main`.
 
 
 ## Finishing steps
@@ -270,8 +270,9 @@ add to the snap.
 
 ### Endless possibilities
 
-Combining various plugins and parts and using the wiki plugin make `snapcraft`
-incredibly versatile. On top of that, you can write [your own plugin] [plugin]
-as well.
+Combining various plugins and parts and using [the wiki][wiki] plugin make
+`snapcraft` incredibly versatile. On top of that, you can write
+[your own plugin] [plugin] as well.
 
 [plugin]: plugins.md
+[wiki]: https://wiki.ubuntu.com/snapcraft/parts

docs/snapcraft-parts.md

@@ -164,3 +164,43 @@ If you only need to embed a Java runtime, add a part with the jdk type. This
 will pull a relocatable OpenJDK via the default-jdk Ubuntu package and will
 set the proper `JAVA_HOME` and `PATH` environment variables in wrapper
 scripts generated by snapcraft or when running the app locally.
+
+
+## Reusing parts
+
+Successful snapcraft parts can be published in
+[the wiki][wiki] and reused by other snapcraft.yaml files.  A part that does
+not include the 'plugin' option will be searched for on [the wiki][wiki] and
+the content from there will be used to build the part locally.  As the number
+of published parts increases, writing a snapcraft.yaml file will get easier
+because one would simply include a set of published parts and the local part(s)
+specific to the snap being created.
+
+The format of the wiki entries is a stream of YAML documents with the following
+required fields:
+
+ * origin - The url where the snapcraft.yaml is located
+ * project-part - The actual part to be published
+ * description - A brief description of the part.
+ * maintainer - The person who maintains the part's name and email address.
+
+Optional fields are:
+
+ * origin-type - A hint about the type of project that contains the part. (e.g. bzr, git, tar)
+ * parts - A YAML list of other parts from the snapcraft.yaml that are needed to use the project-part.
+
+### Note:
+
+It can take up to an hour for updates to the wiki to become available to snapcraft.
+
+### Example:
+
+```yaml
+---
+project-part: curl
+origin: https://github.com/curl/curl
+maintainer: John Doe <john.doe@example.com>
+parts: [libcurl]
+```
+
+[wiki]: https://wiki.ubuntu.com/snapcraft/parts