Add setup/build instructions to CONTRIBUTING.md

Author: nkubala

CONTRIBUTING.md

@@ -3,6 +3,40 @@
 We'd love to accept your patches and contributions to this project. There are
 just a few small guidelines you need to follow.
 
+## Setup
+
+If you haven't written Go code on your machine before, first follow the official
+Golang instructions for setting up your environment: https://golang.org/doc/code.html
+
+Once you have your environment set up, create a fork of the container-diff repository
+with your personal GitHub account. Then, clone the fork into your `$GOPATH`:
+
+```bash
+git clone [email protected]:<your_account>/container-diff.git 
+$GOPATH/src/github.com/GoogleCloudPlatform &&
+cd $GOPATH/src/github.com/GoogleCloudPlatform/container-diff &&
+git remote add upstream [email protected]:GoogleCloudPlatform/container-diff.git
+```
+
+The last command here sets the official repository as an upstream repository for
+your fork, so you can keep your fork in sync with `MASTER`:
+
+```bash
+(container-diff) git pull upstream master && git push origin master
+```
+
+## Building
+
+From the project root, run `make clean && make`. Under the hood, this essentially
+just runs a `go build` on the project root to create an executable in `/out`.
+
+## Code reviews
+
+All submissions, including submissions by project members, require review. We
+use GitHub pull requests for this purpose. Consult
+[GitHub Help](https://help.github.com/articles/about-pull-requests/) for more
+information on using pull requests.
+
 ## Contributor License Agreement
 
 Contributions to this project must be accompanied by a Contributor License
@@ -15,13 +49,6 @@ You generally only need to submit a CLA once, so if you've already submitted one
 (even if it was for a different project), you probably don't need to do it
 again.
 
-## Code reviews
-
-All submissions, including submissions by project members, require review. We
-use GitHub pull requests for this purpose. Consult
-[GitHub Help](https://help.github.com/articles/about-pull-requests/) for more
-information on using pull requests.
-
 ## Tests
 
 Before sending a PR, please make sure the included tests pass.