update readme for the build instructions

Author: c9s

README.md

@@ -3,4 +3,24 @@
 See phpbrew/phpbrew on Docker Hub <https://hub.docker.com/r/phpbrew/phpbrew/tags>
 
 
+## Build
+
+The build script already provides the magic to build all the images:
+
+```sh
+./build
+```
+
+To build the phpbrew base image only:
+
+```
+./build phpbrew-base
+```
+
+
+To build a specific image only:
+
+```
+./build phpbrew/18.04-minimum
+```