Add root disk partitions docs

This introduces a new user guide section and adds root disk
partitions docs

Change-Id: I3b2411e8d02f6a3430d0f527f44c28dd2f780012

doc/source/index.rst

@@ -81,6 +81,18 @@ Sample Policy file
 
     sample_policy
 
+
+User's Guide
+============
+
+Root Disk Partitions
+--------------------
+
+.. toctree::
+    :maxdepth: 1
+
+    user/root-disk-partitions
+
 Command References
 ==================
 
@@ -91,11 +103,6 @@ Here are references for commands not elsewhere documented.
 
   cmds/mogan-dbsync
 
-Indices and tables
-==================
-
-* :ref:`search`
-
 Running tests of Mogan
 ======================
 
@@ -112,3 +119,8 @@ Installation and Setup
 
    installation/uwsgi
    installation/image-requirements
+
+Indices and tables
+==================
+
+* :ref:`search`

doc/source/user/root-disk-partitions.rst

@@ -0,0 +1,13 @@
+Root disk partitions
+--------------------
+
+Mogan supports custom partitions for root disk with partition images, meaning
+that users can specify partitions such as root_gb, ephemeral_gb, and swap_mb
+when claiming a server. But after the deployment the root partition may not
+like what your specified, as the backend driver makes the root partition the
+last partition which enables tools like cloud-init's growroot utility to expand
+the root partition until the end of the disk.
+
+.. note:: Whole disk images, on the contrary, not support partitions, passing
+          partitions to server creation results in a fault and will prevent the
+          creation from happening.