update README.md with info about possible properties while building

README.md

 Prerequisites
 =============
 
-In order to build the client-singer (a derivate of the Open eCard project with an ReQESIDTA addon) some additional tools are needed.
+In order to build the eID-Clien aka. client-singer (a derivate of the Open eCard project with an ReQESIDTA addon) some additional tools are needed.
 
 Required dependencies are:
+
 * Java JDK 11 or higher - Oracle JDK and OpenJDK are working correctly (jlink is required for building modular runtime images)
 
-* Maven in at least version 3.6.1
+* Maven in at least version 3.6.1 (https://maven.apache.org/download.html)
+
+Configure
+=========
+
+If the client should communicate with an eID-Server (this means the ReQESIDTA stack) using an self signed certificate in the reverse proxy,
+the properties file `addons/reqesidta/src/main/resources/reqesidta/default.properties` must be edited before building.
 
-  https://maven.apache.org/download.html
+* Change the value of 'validate-server-cert' from 'true' to 'false'.
 
 Build Sources
 =============
 
 A standard build is performed by the command:
 
-    $ mvn clean install
+  $ mvn clean install
 
 Run the client
 ==============