add build doc

It is useful for build, since the clone dir must be
`$GOPATH/src/helm.sh/helm.`

Signed-off-by: Xiang Dai <764524258@qq.com>

docs/developers.md

@@ -0,0 +1,27 @@
+# Build Guide
+
+This guide explains how to set up your environment for developing on
+Helm and Tiller.
+
+## Prerequisites
+
+- The latest version of Go
+- The latest version of Glide
+- Git
+
+## Building Helm/Tiller
+
+We use Make to build our programs. The simplest way to get started is:
+
+```console
+$ cd $GOPATH
+$ mkdir -p src/helm.sh
+$ cd src/helm.sh
+$ git clone https://github.com/helm/helm.git
+$ cd helm
+$ make build
+```
+
+NOTE: If not running from the path `$GOPATH/src/helm.sh/helm`, build would fail or `helm` binary is not work with own code. The
+directory `k8s.io` should not be a symlink or `build` will not find the relevant
+packages.