Ussuri contributor docs community goal

- Updated CONTRIBUTING.rst per goal guidelines
- Created doc/source/contributor/contributing.rst per goal guidelines

Change-Id: I7b8f6bd655b607e403748481b76b1adbfb6e26af

CONTRIBUTING.rst

@@ -0,0 +1,19 @@
+The source repository for this project can be found at:
+
+   https://opendev.org/openstack/sahara-plugin-vanilla
+
+Pull requests submitted through GitHub are not monitored.
+
+To start contributing to OpenStack, follow the steps in the contribution guide
+to set up and use Gerrit:
+
+   https://docs.openstack.org/contributors/code-and-documentation/quick-start.html
+
+Bugs should be filed on Storyboard:
+
+   https://storyboard.openstack.org/#!/project/openstack/sahara-plugin-vanilla
+
+For more specific information about contributing to this repository, see the
+sahara-plugin-vanilla contributor guide:
+
+   https://docs.openstack.org/sahara-plugin-vanilla/latest/contributor/contributing.html

doc/source/contributor/contributing.rst

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+============================
+So You Want to Contribute...
+============================
+
+For general information on contributing to OpenStack, please check out the
+`contributor guide <https://docs.openstack.org/contributors/>`_ to get started.
+It covers all the basics that are common to all OpenStack projects: the
+accounts you need, the basics of interacting with our Gerrit review system, how
+we communicate as a community, etc.
+
+sahara-plugin-vanilla is maintained by the OpenStack Sahara project.
+To understand our development process and how you can contribute to it, please
+look at the Sahara project's general contributor's page:
+http://docs.openstack.org/sahara/latest/contributor/contributing.html

doc/source/contributor/index.rst

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+=================
+Contributor Guide
+=================
+
+.. toctree::
+   :maxdepth: 2
+
+   contributing

doc/source/index.rst

@@ -5,3 +5,4 @@ Vanilla plugin for Sahara
    :maxdepth: 2
 
    user/index
+   contributor/index