docs: update readme

Author: saitho

README.md

@@ -4,4 +4,25 @@ StackHead module for using Nginx as webserver.
 
 ## Installation
 
-TBD
+Install it via `ansible-galaxy`:
+
+```
+ansible-galaxy install getstackhead.stackhead_module_webserver_nginx
+```
+
+In order to use Nginx with [StackHead](https://get.stackhead.io), set `stackhead__webserver` it in your inventory file:
+
+```yaml
+# inventory for integration test
+---
+all:
+  vars:
+    # Use Nginx as webserver
+    stackhead__webserver: getstackhead.stackhead_module_webserver_nginx
+  hosts:
+    myserver:
+      ansible_host: 123.456.789 # ...
+      stackhead:
+        applications:
+          # ...
+```