Merge branch 'deploy-key-doc' into 'master'

Deploy Key Doc

/cc @marin @job @jacobvosmaer

doc/ssh/deploy_keys.md

+Deploy keys allow read-only access one or multiple projects with a single SSH key.
+
+This is really useful for cloning repositories to your Continuous Integration (CI) server.
+By using a deploy keys you don't have to setup a dummy user account.
+
+If you are a project master or owner you can add a deploy key in the project settings under the section Deploy Keys.
+Press the 'New Deploy Key' button and upload a public ssh key.
+After this the machine that uses the corresponding private key has read-only access to the project.
+
+You can't add the same deploy key twice with the 'New Deploy Key' option.
+If you want to add the same key to another project please enable it in the list that says 'Deploy keys from projects available to you'.
+You need to be the owner of the deploy key to see it in this list.