Add link to System Lambda to Readme

Make people aware of this alternative. It is very helpful for developers
that use JUnit Jupiter or TestNG because they cannot use System Rules.

Author: stefanbirkner

README.md

@@ -5,6 +5,12 @@
 System Rules is a collection of JUnit rules for testing code which uses
 `java.lang.System`.
 
+[System Lambda](https://github.com/stefanbirkner/system-lambda) is an
+alternative to System Rules that leverages the possibilities of Java 8. It is
+independent of the test framework. You can use it for example as a replacement
+for System Rules in [JUnit Jupiter](https://junit.org/junit5/) and
+[TestNG](https://testng.org/doc/index.html).
+
 
 ## Installation