docs: clarify that path must be a directory (#142)

The `path` parameter is passed to the systemd-journal gem's `:path` option,
which opens journal files in a directory only.
A glob or individual file (e.g. `/var/log/journal/*.journal`) does not work and fails silently with
no logs read.

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>

Author: Watson1978

README.md

@@ -69,7 +69,9 @@ If you are upgrading to version 1.0 from a previous version of this plugin take
 
 **`path`**
 
-Path to the systemd journal, defaults to `/var/log/journal`
+Path to the systemd journal **directory**, defaults to `/var/log/journal`.
+
+This must be a directory, not an individual file or a glob pattern — for example, `/var/log/journal/*.journal` will not work. All journal files in the given directory are read automatically.
 
 **`filters`**