README.md: Inform about -t command line option

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@@ -173,6 +173,27 @@ command line option. For example:
 slcl -p 7822 ~/my-db/
 ```
 
+`slcl` requires a temporary directory where files uploaded by users are
+temporarily stored until moved to the user directory. By default, `slcl`
+attempts to retrieve the path to the temporary directory by inspecting the
+`TMPDIR` environment variable, and falls back to `/tmp` (which is common
+among Unix-like operating systems such as GNU/Linux, although not defined
+by POSIX) if undefined.
+
+If a custom temporary directory is required, it can be defined via command
+line option `-t`. For example:
+
+```sh
+slcl -t ~/my-tmp -p 7822 ~/my-db
+```
+
+**Note:** system-level temporary directories such as `/tmp` might reside
+on a filesytem different than the one where the database resides. This
+would force `slcl` to copy the contents from uploaded files from the
+temporary directory to the database, which might be an expensive operation.
+Therefore, in order to avoid expensive copies, define a custom temporary
+directory that resides on the same filesystem.
+
 ## Why this project?
 
 Previously, I had been recommended Nextcloud as an alternative to proprietary