Added build doc

This commit added a build chapter into the doc to explain how to compile
`containerd` binaries via Docker container.

I did that after a discussion on Slack. For some reason in my Ubuntu
16.10 I am not able to build containerd. It's probably some trick mess
that I created (I am super powerful on these things).o

But it can be useful for other people like me!

Signed-off-by: Gianluca Arbezzano <gianarb92@gmail.com>

BUILDING.md

@@ -0,0 +1,71 @@
+# Build containerd from source
+
+This guide is useful if you intend to contribute on containerd. Thanks for your
+effort. Every contribution is very appreciated.
+
+## Build the development environment
+
+In first you need to setup your Go development environment. You can follow this
+guideline [How to write go code](https://golang.org/doc/code.html) and at the
+end you need to have `GOPATH` and `GOROOT` set in your environment.
+
+At this point you can use `go` to checkout `containerd` in your `GOPATH`:
+
+```sh
+go get github.com/containerd/containerd
+```
+
+`containerd` uses [Btrfs](https://en.wikipedia.org/wiki/Btrfs) it means that you
+need to satisfy this dependencies in your system:
+
+* CentOS/Fedora: `yum install btrfs-progs-devel`
+* Debian/Ubuntu: `apt-get install btrfs-tools`
+
+At this point you are ready to build `containerd` yourself.
+
+## In your local environment
+
+`containerd` uses `make` to create a repeatable build flow. It means that you
+can run:
+
+```sudo
+make
+```
+
+This is going to build all the binaries provided by this project in the `./bin`
+directory.
+
+You can move them in your global path with:
+
+```sudo
+sudo make install
+```
+
+## Via Docker Container
+
+You can build `containerd` via Docker container. You can build an image from
+this `Dockerfile`:
+
+```
+FROM golang
+
+RUN apt-get update && \
+    apt-get install btrfs-tools
+```
+
+Let's suppose that you built an image called `containerd/build` and you are into
+the containerd root directory you can run the following command:
+
+```sh
+docker run -it --rm \
+    -v ${PWD}:/go/src/github.com/containerd/containerd \
+    -e GOPATH=/go \
+    -w /go/src/github.com/containerd/containerd containerd/build make
+```
+
+At this point you can find your binaries in the `./bin` directory in your host.
+You can move the binaries in your `$PATH` with the command:
+
+```sh
+sudo make install
+```