Add libravatar documentation.

doc/README.md

@@ -20,6 +20,7 @@
 - [Update](update/README.md) Update guides to upgrade your installation.
 - [Welcome message](customization/welcome_message.md) Add a custom welcome message to the sign-in page.
 - [Issue closing](customization/issue_closing.md) Customize how to close an issue from commit messages.
+- [Libravatar](customization/libravatar.md) Use Libravatar for user avatars.
 ## Contributor documentation

doc/customization/libravatar.md

+# Use Libravatar service with GitLab
+
+GitLab by default supports [Gravatar](gravatar.com) avatar service.
+Libravatar is a service which delivers your avatar (profile picture) to other websites and their API is
+[heavily based on gravatar](http://wiki.libravatar.org/api/).
+
+This means that it is not complicated to switch to Libravatar avatar service or even self hosted Libravatar server.
+
+# Configuration
+
+In [gitlab.yml gravatar section](https://gitlab.com/gitlab-org/gitlab-ce/blob/672bd3902d86b78d730cea809fce312ec49d39d7/config/gitlab.yml.example#L122) set
+the configuration options as follows:
+
+## For HTTP
+
+```yml
+  gravatar:
+    enabled: true
+    # gravatar urls: possible placeholders: %{hash} %{size} %{email}
+    plain_url: "http://cdn.libravatar.org/avatar/%{hash}?s=%{size}&d=identicon"
+```
+
+## For HTTPS
+
+```yml
+  gravatar:
+    enabled: true
+    # gravatar urls: possible placeholders: %{hash} %{size} %{email}
+    ssl_url: "https://seccdn.libravatar.org/avatar/%{hash}?s=%{size}&d=identicon"
+```
+
+## Self-hosted
+
+If you are [running your own libravatar service](http://wiki.libravatar.org/running_your_own/) the url will be different in the configuration
+but the important part is to provide the same placeholders so GitLab can parse the url correctly.
+
+For example, you host a service on `http://libravatar.example.com` the `plain_url` you need to supply in `gitlab.yml` is
+
+`http://libravatar.example.com/avatar/%{hash}?s=%{size}&d=identicon`
+
+
+## Omnibus-gitlab example
+
+In `/etc/gitlab/gitlab.rb`:
+
+#### For http
+
+```ruby
+gitlab_rails['gravatar_enabled'] = true
+gitlab_rails['gravatar_plain_url'] = "http://cdn.libravatar.org/avatar/%{hash}?s=%{size}&d=identicon"
+```
+
+#### For https
+
+```ruby
+gitlab_rails['gravatar_enabled'] = true
+gitlab_rails['gravatar_ssl_url'] = "https://seccdn.libravatar.org/avatar/%{hash}?s=%{size}&d=identicon"
+```
+
+
+Run `sudo gitlab-ctl reconfigure` for changes to take effect.
+
+
+## Default URL for missing images
+
+[Libravatar supports different sets](http://wiki.libravatar.org/api/) of `missing images` for emails not found on the Libravatar service.
+
+In order to use a different set other than `identicon`, replace `&d=identicon` portion of the url with another supported set.
+For example, you can use `retro` set in which case url would look like: `plain_url: "http://cdn.libravatar.org/avatar/%{hash}?s=%{size}&d=retro"`