Adding section "Building FreeIPA from source" on README

https://pagure.io/freeipa/issue/6725

Reviewed-By: Tomas Krizek <tkrizek@redhat.com>
Reviewed-By: Rob Crittenden <rcritten@redhat.com>

README.md

@@ -55,6 +55,12 @@ http://freeipa.org/page/Documentation .
 To get started quickly, start here:
 http://www.freeipa.org/page/Quick_Start_Guide
 
+## For developers
+
+* Building FreeIPA from source
+    * http://www.freeipa.org/page/Build
+    * See the BUILD.txt file in the source root directory
+ 
 ## Licensing
 
 Please see the file called COPYING.