start writing deploy key to multiple projects

doc/api/deploy_key_multiple_projects.md

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+# Adding deploy keys to multiple projects
+
+If you want to easily add the same deploy key to multiple projects in the same group, this can be achieved quite easily with the API.
+
+First, find the ID of the projects you're interested in, by either listing all projects:
+
+```
+curl https://gitlab.com/api/v3/projects?private_token=abcdef
+```
+
+Or finding the id of a group and then listing all projects in that group:
+
+```
+curl https://gitlab.com/api/v3/groups?private_token=abcdef
+
+curl https://gitlab.com/api/v3/groups/1234?private_token=abcdef # where the id of the group is 1234
+```
+
+With those IDs, add the same deploy key to all:
+```
+curl -X POST curl https://gitlab.com/api/v3/projects/321/deploy_key_here?private_token=abcdef
+```