[doc] adding simple usage

doc/usage.qmd

@@ -1,3 +1,11 @@
 ---
 title: How to use it
 ---
+
+If you have a folder or multiple folders with each containing two recordings one from `relacs` and one from `open-ephys` you can simply run the CLI like this:
+
+```{python}
+# leave out the ! at the beginning if you running this in your shell
+!oephys2nix ../oephys2nix/test/Test1/
+```
+which provides you with information about the transition of the stimuli into the new file.