Add documentation on getting the pprof source. (#279)

* Add documentation on getting the source.

* Update the doc more.

CONTRIBUTING.md

@@ -11,6 +11,45 @@ through the issue tracker with your idea so that we can help out and possibly
 guide you. Coordinating up front makes it much easier to avoid frustration later
 on.
 
+# Development
+
+Make sure `GOPATH` is set in your current shell. The common way is to have
+something like `export GOPATH=$HOME/gocode` in your `.bashrc` file so that it's
+automatically set in all console sessions.
+
+To get the source code, run
+
+```
+go get github.com/google/pprof
+```
+
+To run the tests, do
+
+```
+cd $GOPATH/src/github.com/google/pprof
+go test -v ./...
+```
+
+When you wish to work with your own fork of the source (which is required to be
+able to create a pull request), you'll want to get your fork repo as another Git
+remote in the same `github.com/google/pprof` directory. Otherwise, if you'll `go
+get` your fork directly, you'll be getting errors like `use of internal package
+not allowed` when running tests.  To set up the remote do something like
+
+```
+cd $GOPATH/src/github.com/google/pprof
+git remote add aalexand git@github.com:aalexand/pprof.git
+git fetch aalexand
+git checkout -b my-new-feature
+# hack hack hack
+go test -v ./...
+git commit -a -m "Add new feature."
+git push aalexand
+```
+
+where `aalexand` is your Github user ID. Then proceed to the Github UI to send a
+code review.
+
 # Code reviews
 
 All submissions, including submissions by project members, require review.