[gimp-help] Update README to explain how to get PACKAGE_VERSION replaced in msginit

[Jacob Boerema <jboerema src gnome org>]

commit 4b70b3427000d33a320de8bf360e9a5c28dd048a
Author: Jacob Boerema <jgboerema gmail com>
Date:   Tue Mar 29 14:31:10 2022 -0400

    Update README to explain how to get PACKAGE_VERSION replaced in msginit

 README.md | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/README.md b/README.md
index a50116d3f..07a28564b 100644
--- a/README.md
+++ b/README.md
@@ -354,6 +354,12 @@ of languages needs to be updated in the following files:
     - TODO: Decide if we want to move this whole chapter to TRANSLATING,
       or the reverse, or ...
 
+Note: If your build directory is not a child of the source directory, then
+`msginit` will not fill in PACKAGE_VERSION in the header of the po files.  
+Since msginit tries to find `configure`, copying that to your build directory
+can solve that issue. In that case, make sure to update it when switching
+branches.
+
 When all of the above are updated, run:
   - `make po` in the main build directory
   - `make po` in the quickreference directory