Updates README

Author: tomsteele

README.md

@@ -10,3 +10,36 @@ $ export MONGO_URL=mongodb://localhost:27017/lair
 $ export API_LISTENER=ip:port
 $ ./api-server
 ```
+
+## Transforms
+Transforms allow you to use Go plugins to modify a project that is sent as part of an update. Go plugins are Linux only and have been available since v1.8.
+
+### Confguration
+The api-server will load all plugins from a directory that is set via the `TRANSFORM_DIR` environment variable. Example:
+```
+$ export TRANSFORM_DIR=plugins
+$ ls plugins
+iis.so foo.so bar.so
+$ ./api-server
+2017/03/03 12:56:05 Loading transform iis.so
+2017/03/03 12:56:05 Loading transform foo.so
+2017/03/03 12:56:05 Loading transform bar.so
+````
+
+### Writing Transforms
+Writing transform involves implmenting a single function, `Update(p *lair.Project`. Here is a stub.
+```
+package main
+
+import (
+    lair "github.com/lair-framework/go-lair"
+)
+
+func Update(p *lair.Project) {
+    for i := range p.Hosts {
+        p[i].Host.OS = "Trash"
+    }
+}
+```
+
+This can now be built by running `go build -buildmode=plugin`.