Updates README with conflict prevention info.

Author: aaronj-ep

README.markdown

@@ -4,6 +4,14 @@ PropelBundle
 This an unofficial implementation of [Propel 1.6](http://www.propelorm.org/) in Symfony, which has been modified to work
 with PHP 7.2+ and Symfony 3.0+.
 
+## Preventing Conflicts
+
+To prevent conflicts with a non-patched versions of Propel1, it is recommended to add the following to the `autoload` key of your project's `composer.json`:
+
+    "exclude-from-classmap": [ "vendor/propel/propel1/runtime/lib","vendor/propel/propel1/generator/lib" ]
+
+These conflicts are created when a dependency you have specified has `propel/propel1` in their `required-dev` packages, and therefore cannot be otherwise avoided.
+
 ## Branching model
 
 ### Propel1 integration