First draft of pages documentation

doc/pages/README.md

 # GitLab Pages
 
-To start using GitLab Pages add to your project .gitlab-ci.yml with special pages job.
+_**Note:** This feature was introduced in GitLab EE 8.3_
 
-    pages:
-      image: jekyll
-      script: jekyll build
-      artifacts:
-        paths:
-        - public
+To start using GitLab Pages add to your project `.gitlab-ci.yml` the special
+`pages` job. The example below is using [jekyll][] and assumes the created
+HTML files are generated under the `public/` directory which resides under the
+root directory of your Git repository.
 
-TODO
+```yaml
+pages:
+  image: jekyll
+  script: jekyll build
+  artifacts:
+    paths:
+    - public
+```
+
+- The pages are created when build artifacts for `pages` job are uploaded
+- Pages serve the content under: http://group.pages.domain.com/project
+- Pages can be used to serve the group page, special project named as host: group.pages.domain.com
+- User can provide own 403 and 404 error pages by creating 403.html and 404.html in group page project
+- Pages can be explicitly removed from the project by clicking Remove Pages in Project Settings
+- The size of pages is limited by Application Setting: max pages size, which limits the maximum size of unpacked archive (default: 100MB)
+- The public/ is extracted from artifacts and content is served as static pages
+- Pages asynchronous worker use `dd` to limit the unpacked tar size
+- Pages needs to be explicitly enabled and domain needs to be specified in gitlab.yml
+- Pages are part of backups
+- Pages notify the deployment status using Commit Status API
+- Pages use a new sidekiq queue: pages
+- Pages use a separate nginx config which needs to be explicitly added
+
+## Examples
+
+- Add example with stages. `test` using a linter tool, `deploy` in `pages`
+- Add examples of more static tool generators
+
+```yaml
+image: jekyll
+
+stages:
+  - test
+  - deploy
+
+lint:
+  script: jekyll build
+  stage: test
+
+pages:
+  script: jekyll build
+  stage: deploy
+  artifacts:
+    paths:
+    - public
+```
+
+## Current limitations
+
+- We currently support only http and port 80. It will be extended in the future.