README: add repl instructions

README.md

@@ -41,3 +41,16 @@ scritcher examples/noise.scri blah.bmp
 // blah_g1.bmp, the second saves to blah_g2.bmp, etc.
 $your_image_viewer blah_g1.bmp
 ```
+
+# using the repl
+
+using repl works via `scritcher repl scri_file.scri input_image.bmp`
+
+you type commands as you'd write the specific scritcher commands
+(`doc/README.md`), with three repl-specific ones:
+ - `push`, to push the last written command to the queue
+ - `save`, to write the queue to the given `scri_file.scri` file
+ - `list`, to print the current contents of the queue
+
+this allows for quicker iteration of commands, as you can type a command, tweak
+its arguments, and when satisfied, `push` it, and work on the next one, etc.