add repo location of unit test code

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1784504 13f79535-47bb-0310-9956-ffa450edef68

README.rst

 * Check out the code from svn or git
 * Import the project into Eclipse or your favorite IDE
 * Write a unit test:
+  * Binary formats and Common APIs: src/testcases/org/apache/poi/
+  * OOXML APIs only: src/ooxml/testcases/org/apache/poi/
+  * Scratchpad (Binary formats): src/scratchpad/testcases/org/apache/poi/
+  * test files: test-data/
 * Navigate the source, make changes, and run unit tests to verify
   * Binary formats and Common APIs: src/java/org/apache/poi/
   * OOXML APIs only: src/ooxml/java/org/apache/poi/