Add tutorial for adding custom content from build root

Some users are asking for instructions on how to build custom bundles
without having to go through the pain of packaging their content in an
RPM. Provide a tutorial explaining how this can be achieved.

Signed-off-by: Matthew Johnson <matthew.johnson@intel.com>

docs/tutorials/custom-content-from-chroot.md

@@ -0,0 +1,67 @@
+# Adding Custom Content from Build Root
+
+When generating update content based on an upstream mix (such as upstream Clear
+Linux), it is possible to provide custom bundles (and custom content) without
+using RPMs as input and instead providing a full build root of the content after
+the normal bundle building stage. There are a few steps that need to be
+observed.
+
+First, initialize your workspace and build your bundles as usual. This workflow
+will assume a build based on an upstream mix.
+
+```bash
+mixer init --clear-version <version> --mix-version <mix-version>
+mixer bundle add --all-upstream  # do not add your custom bundle
+sudo mixer build bundles
+```
+
+At this point mixer has created several `bundle-info` files in your
+`<ws>/update/image/<mix-version>/` directory. These files define the package and
+file lists for each bundle that was added in the `mixer bundle add
+--all-upstream` step above. Your custom bundle did not have a `bundle-info` file
+created for it.
+
+Now we will create your custom bundle. This tutorial assumes you have the build
+root containing your custom content on your system already. How you generate
+this is up to you.
+
+Copy your custom content into the `<ws>/update/image/<mix-version>`
+
+```bash
+sudo mkdir <ws>/update/image/<mix-version>/<bundle-name>
+sudo cp -a <path/to/custom/build/root> <ws>/update/image/<mix-version>/<bundle-name>
+```
+
+**WARNING:** it is possible to add files that conflict with files provided by
+upstream. Take care to not introduce conflicting files. A conflicting file is a
+file with the same filename as another file but with different contents,
+permissions, or ownership. **Introducing a conflicting file can break client
+updates.** Before adding your custom content, check for conflicting file names
+in the `<ws>/image/<mix-version>/full` build root. This build root contains all
+files in the mix. A common way to avoid file conflicts is to namespace your
+content under a specific directory, such as `/<bundle-name>/*`.
+
+An example of a conflicting file would be if your content provided a custom
+`/usr/bin/yum` but the file exists in
+`<ws>/image/<mix-version>/full/usr/bin/yum` with different content or
+permissions.
+
+Now update the `<ws>/update/groups.ini` file to add your custom bundle
+(bundle-name) to your bundle list. At the bottom of that file add a new section.
+
+```toml
+[<bundle-name>]
+group=<bundle-name>
+```
+
+Replace \<bundle-name\> with the name of your custom bundle. Remove the \<\>
+brackets.
+
+You can now continue building your update.
+
+```bash
+sudo mixer build update
+```
+
+Mixer will build an update including your custom bundle and custom content you
+provided in the build root.