[simple-scan] Add note about system vs. subproject libhandy

[Bartosz <bkosiorek src gnome org>]

commit 396c1b108838ddb98ce371a4cb0a838e210bbe73
Author: Johannes Schilling <dario deaktualisierung org>
Date:   Wed Jul 21 22:31:46 2021 +0200

    Add note about system vs. subproject libhandy

 README.md | 7 +++++++
 1 file changed, 7 insertions(+)
---
diff --git a/README.md b/README.md
index 7b6e8ee3..0b6751a2 100644
--- a/README.md
+++ b/README.md
@@ -54,6 +54,13 @@ $ ninja -C _build all install
 $ XDG_DATA_DIRS=_install/share:$XDG_DATA_DIRS ./_install/bin/simple-scan
 ```
 
+If libhandy is used as a subproject (i.e. system libhandy not found or
+too old), then make sure to use the resulting .so file as well when
+running, e.g. by using the `LD_LIBRARY_PATH` variable:
+```
+$ LD_LIBRARY_PATH=_install/lib/x86_64-linux-gnu XDG_DATA_DIRS=_install/share:$XDG_DATA_DIRS 
./_install/bin/simple-scan
+```
+
 # Debugging
 
 There is a `--debug` command line switch to enable more verbose logging: