Add build requirements to README

Author: Wolfgang Ginolas

README.md

@@ -102,6 +102,16 @@ git clone https://github.com/cryptpad/nextcloud-open-in-cryptpad.git openincrypt
 
 ## Building the app
 
+### Requirements
+
+On a Ubuntu like distribution you will need these packages:
+
+``` sh
+sudo apt install php-cli composer php-xml
+```
+
+### Run the build
+
 Prior to building, install the requisite dependencies by running:
 
     npm ci