docs(readme): add section to document the build

Signed-off-by: Thierry Bugier <[email protected]>

Author: btry

README.md

@@ -20,6 +20,7 @@ Extend GLPI with Plugins.
 * [Versioning](#versioning)
 * [Contact](#contact)
 * [Professional Services](#professional-services)
+* [Build from source](#build-from-source)
 * [Contribute](#contribute)
 * [Copying](#copying)
 
@@ -74,6 +75,14 @@ The GLPI Network services are available through our [Partner's Network](http://w
 
 Obtain a personalized service experience, associated with benefits and opportunities.
 
+## Build from source
+
+To build the plugin you need [Composer](http://getcomposer.org) and an internet access to download some resources from Github.
+
+After dowloading the source of Formcreator, go in its folder and run the following
+* composer install
+* php vendor/bin/robo build:fa-data
+
 ## Contribute
 
 Want to file a bug, contribute some code, or improve documentation? Excellent! Read up on our