dev (gitattributes): use union merge for ACKNOWLEDGEMENTS and DEVICES

These files are just lists where order is not that important.
DEVICES.md should be in alphabetical order within each device type
section, so rarely conflicts, but ACKNOWLEDGEMENTS.md is appended
with new contributors at the end, so if someone adds their own name
it is almost guaranteed to conflict with default merge settings.

union merge will use both changes in cases where there is a conflict,
which is almost always the right thing for these two files.

.gitattributes

@@ -0,0 +1,2 @@
+ACKNOWLEDGEMENTS.md merge=union
+DEVICES.md merge=union