Document running/debugging from Eclipse

The com.google.gitiles.sourcePath property needs to be set to the
root folder of the Gitiles source tree. The tools/run_dev.sh does
that automatically.

Change-Id: I534bfac10aef63a8d977f38b5d16d0350cd90904

Documentation/developer-guide.md

diff --git a/Documentation/developer-guide.md b/Documentation/developer-guide.md
index 59a36f9..2817dbb 100644
--- a/Documentation/developer-guide.md
+++ b/Documentation/developer-guide.md
@@ -52,6 +52,15 @@
 libraries have changed. For best results, ensure the project is closed in
 Eclipse before rebuilding.
 
+## Running/Debugging from Eclipse IDE
+
+Running Gitiles from Eclipse requires setting the
+`com.google.gitiles.sourcePath` system property. The property value has to be
+the root folder of the Gitiles source code, for example:
+
+````
+-Dcom.google.gitiles.sourcePath=/home/johndoe/git/gitiles
+````
 
 ## Code Style