man: doveadm-fs - Add usage example

Author: cmouse

docs/core/man/include/doveadm-fs.inc

@@ -139,6 +139,13 @@ size (in bytes) of the item is returned.
 
 <!-- @include: ./reporting-bugs.inc -->
 
+## EXAMPLE
+
+How to decompress a file.
+```
+doveadm -o fs=compress,posix -o fs/compress/fs_driver=compress -o fs/posix/fs_driver=posix -o fs_posix_prefix=/tmp/ fs get empty test.gz
+```
+
 ## SEE ALSO
 
 [[man,doveadm]]