Add building from source doc (#204)

* Add building from source doc

* typo

docs/build_from_source.md

+# Building from source
+
+Building a new version of MicroK8s from the source is straightforward.
+
+1.  If you don't already have it, install the latest version of the `snapcraft` tool:
+     ```bash
+     sudo snap install snapcraft --classic
+     ```
+1. Clone the MicroK8s repository:
+     ```bash
+     git clone https://github.com/ubuntu/microk8s.git
+     cd microk8s/
+     ```
+1. Ensure local package lists are up to date. On Ubuntu, run:
+     ```bash
+     sudo apt update
+     ```
+1. Run snapcraft to build the snap package:
+     ```bash
+     snapcraft
+     ```
+1. Once the snap is built it can be installed with:
+     ```bash
+     snap install microk8s_v1.12.2_amd64.snap --classic --dangerous
+     ```
+     (substitute the name of the version that was actually built as required).
+
+For more information on managing snaps, see the [Snapcraft documentation](https://docs.snapcraft.io/getting-started/3876)
+
+
+## Using different build options
+
+You can set the following environment variables prior to building:
+
+| Variable                 | Description                                                    | Default value                     |
+|--------------------------|-----------------------------------------------------------|-------------------------------------|
+| KUBE_VERSION  | Kubernetes release to package               | Latest stable version       |
+| ETCD_VERSION  | Version of etcd                                              | 3.3.4                                       |
+| CNI_VERSION      | Version of CNI tools                                    | 0.7.1                                       |
+| KUBE_TRACK       | Kubernetes series (e.g., 1.10) to package | Latest stable                 |
+| ISTIO_VERSION   | istio release                                                  | v1.0.0                                     |
+
+For example:
+
+```bash
+KUBE_VERSION=v1.9.6 snapcraft
+```
+
+## Faster builds
+
+To speed-up the build process you can reuse the binaries already downloaded from a
+previous build. Binaries are placed under `parts/microk8s/build/build/kube_bins`.
+All you need to do is to make a copy of this directory and have the `KUBE_SNAP_BINS`
+environment variable point to it.
+
+After a standard build, run the following:
+
+```bash
+cp -r parts/microk8s/build/build/kube_bins .
+export KUBE_SNAP_BINS=$PWD/kube_bins/v1.12.2/
+snapcraft
+ ```
+
+...this build will be much faster and will reuse binaries in `KUBE_SNAP_BINS`