Documentation: Standalone binary / pyinstaller extracts dependencies into /tmp. Currently /tmp requires about ~28MB of free space. It also needs exec permissions.

Closes #499

docs/installation.rst

@@ -69,6 +69,8 @@ make borg readable and executable for its users and then you can run ``borg``::
     sudo chown root:root /usr/local/bin/borg
     sudo chmod 755 /usr/local/bin/borg
 
+Note that the binary uses /tmp to unpack |project_name| with all dependencies. It will fail if /tmp has not enough free space or is mounted with the ``noexec`` option. You can change the temporary directory by setting the ``TEMP`` environment variable before running |project_name|.
+
 If a new version is released, you will have to manually download it and replace
 the old version using the same steps as shown above.