add documentation

doc/README.md

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+scritcher command reference
+---
+
+scritcher scripts are shell-like commands separated by a semicolon.
+
+## parameters of the arg type
+
+some commands allow a `:N` syntax where N is an integer to use a given
+argument when `scritcher` is called. so, for example, calling a script:
+```
+load :1;
+showpath;
+```
+
+and then doing `scritcher path/to/script.scri arg` will print `arg` on the
+screen and then exit.
+
+
+## `load path_or_arg`
+
+Load a file into memory. The file will be converted to BMP.
+
+## `echo split index delay`
+
+Run an echo filter on the given loaded file.
+
+## `quicksave`
+
+Save the file on the same directory of the file specified by `load`, but
+with a suffix on the filename (before extension).
+
+Doing consecutive `quicksave`s will not overwrite any files, the suffixes will
+just be different.
+
+## `showpath`
+
+Print the current loaded path on the screen.