Adjust the README with instructions for first time Gophers

This commit adds instructions on how to checkout the code on your
computer and build.

This is needed to avoid friction with executing `got get` pointing to a
fork.

When a new contributor tries to `go get` from their fork, they will be
welcomed with an error message due to the import path still pointing to
the canonical project.

This commit adds the necessary information on how to checkout the
project and configure the remote as instructed by the community.

README.md

@@ -219,6 +219,25 @@ add your name and email address to the
 under the "Contributors" section using the format:
 `First Last <[email protected]>`.
 
+First, to setup the project, ensure that `$GOPATH` variable is pointing to an
+existing directory, where all your Go code will be available.
+
+After forking the repo on Github, run:
+
+```sh
+go get github.com/digitalocean/do-agent
+cd $GOPATH/src/github.com/digitalocean/do-agent
+
+git remote add my-fork <the url for your fork>
+make test # ensure tests are passing
+```
+
+Push your changes to your fork:
+
+```sh
+git push -u master my-fork
+```
+
 Finally, submit a pull request for review!
 
 ## Report a bug