docs: add install instructions using go install (#11)

Including docs on how to configure for usage with private repositories.

README.md

@@ -1,3 +1,15 @@
+<!-- mdtocstart -->
+
+# Table of Contents
+
+- [terrastack](#terrastack)
+    - [Installing](#installing)
+        - [Go/Git configuration for private Repositories](#gogit-configuration-for-private-repositories)
+    - [Why using stacks?](#why-using-stacks)
+    - [Detecting IaC changes](#detecting-iac-changes)
+
+<!-- mdtocend -->
+
 # terrastack
 
 Terrastack is a tool for managing multiple terraform stacks.
@@ -19,6 +31,48 @@ don't have the `provider` explicitly set, is not runnable hence it's
 So, if your runnable terraform module creates your whole infrastructure, *it's
 also not a stack*.
 
+
+## Installing
+
+To install **terrastack** using Go just run:
+
+```
+go install github.com/mineiros-io/terrastack/cmd/terrastack@<version>
+```
+
+Where **<version>** is any terrastack [version tag](https://github.com/mineiros-io/terrastack/tags),
+or if you are feeling adventurous you can just install **latest**:
+
+```
+go install github.com/mineiros-io/terrastack/cmd/terrastack@latest
+```
+
+We put great effort into keeping the main branch stable, so it should be safe
+to use **latest** to play around, but not recommended for long term automation
+since you won't get the same build result each time you run the install command.
+
+
+### Go/Git configuration for private repositories
+
+While this repository is private, there is some extra work in order to
+download and install it using **go install**. There is two main steps.
+First you need to configure git to use ssh instead of https:
+
+```
+git config --global [email protected]:.insteadOf https://github.com/
+```
+
+This only needs to be done once and will change the **.gitconfig** on your
+host. Then you should always export **GOPRIVATE** for our mineiros-io repos
+before running go install:
+
+```
+export GOPRIVATE=github.com/mineiros-io
+```
+
+More info [here](https://golang.org/ref/mod#private-module-proxy-direct).
+
+
 ## Why using stacks?
 
 The stack concept is advised for several reasons: