Merge pull request #336 from wluxion/patch-1

Update installing-cli.md to mention python-dev for pip install

docker-cloud/installing-cli.md

@@ -42,6 +42,8 @@ Open your terminal or command shell and execute the following command:
 ```bash
 $ pip install docker-cloud
 ```
+If you encounter errors on Linux machines, make sure that `python-dev` is installed.
+For example, on Ubuntu, run the following command: `apt-get install python-dev`
 
 #### Install on macOS