Add note about changes to the Eclipse .project file

If changes are made to the .project file, they will be overwritten
when `buck build eclipse` is run again.

This should be obvious, but I just got bitten by it.

Add a short note about this in the documentation.

Change-Id: Ib9ef8fa2c3b8b5dab49c89ea8d039bd4d18678aa
1 file changed