Add proxy notes to new README.dev.md troubleshooting section.

[with some fixups by tabbott]

README.dev.md

@@ -500,3 +500,22 @@ Possible testing issues
   ][lxc-sf].
 
 [lxc-sf]: https://github.com/fgrehm/vagrant-lxc/wiki/FAQ#help-my-shared-folders-have-the-wrong-owner)
+
+Troubleshooting development environment setup
+=============================================
+
+If you are building the development environment on a network where a
+proxy is required to access the Internet, you will need to set the
+proxy in the environment as follows:
+
+- On Ubuntu, set the proxy environment variables using:
+ ```
+ export https_proxy=http://proxy_host:port
+ export http_proxy=http://proxy_host:port
+ ```
+
+- And set the npm proxy and https-proxy using:
+ ```
+ npm config set proxy http://proxy_host:port
+ npm config set https-proxy http://proxy_host:port
+ ```