Add FAQ and information about creating a banner image

FAQ.md

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+## How do I create a banner to promote my OpenStreetMap event?
+
+We occasionally display banner images on the main page of [openstreetmap.org](https://www.openstreetmap.org/) to
+promote a large OpenStreetMap focused conference or host a worldwide donation
+drive.  This is a great way to reach a lot of people!
+
+1. Please review [openstreetmap/operations#150](https://github.com/openstreetmap/operations/issues/150) to
+know whether your event qualifies for a front-page banner.
+
+2. Create the banner image.  It should be 350px wide and saved in a standard
+web image format (.gif, .jpg, .png).  Also note that a close 'X' will be drawn
+over the top right 60x60px corner of the banner, so please do not put anything
+important in that space.
+
+3. Prepare a pull request which includes the following:
+  * The banner should be saved under the
+  [`app/assets/images/banners/`](https://github.com/openstreetmap/openstreetmap-website/tree/master/app/assets/images/banners) folder, and should have a name that makes it clear what it is for (e.g. `donate-2017.jpg`)
+  * Edit [`config/banners.yml`](https://github.com/openstreetmap/openstreetmap-website/blob/master/config/banners.yml)
+  to contain an entry for the event banner.  It should contain the following:
+    * `id` - a unique identifier (e.g. `donate2017`)
+    * `alt` - alt name for the image (e.g. `OpenStreetMap Funding Drive 2017`)
+    * `link` - URL for your event page (e.g. `https://donate.openstreetmap.org/`)
+    * `img` - the filename for the banner image (e.g. `banners/donate-2017.jpg`)
+    * `enddate` - the final date that the banner will be shown (e.g. `2017-oct-31`)
+  * (optional) Feel free to cleanup the old images from the `app/assets/images/banners/`
+  folder and old entries in the `config/banners.yml` file.
+
+See [PR #1296](https://github.com/openstreetmap/openstreetmap-website/pull/1296)
+as an example.