OCaml: Add a README with a configuration grammar

ocaml/README.md

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+## Configuration parser
+
+Grammar:
+
+    assignment  = { space }, key, { space }, equal, { space }, value, "\n"
+    space       = " " | "\t"
+    key         = letter, { letter | digit | "_" }
+    equal       = "="
+    valuable    = ( letter | digit | "_" | "-" | "~" | "/" ), { valuable }
+    value       = valuable, { " " | valuable }
+
+Written using the ISO 14977 EBNF Notation <https://www.cl.cam.ac.uk/~mgk25/iso-14977.pdf>. In this grammar, `digit` implies `decimal digit`.
+
+See also:
+    - Comparison of BNF notations: <https://www.cs.man.ac.uk/~pjj/bnf/ebnf.html>
+    - W3C ABNF Notation: <https://www.w3.org/Notation.html>
+    - IETF RFC 5234 ABNF Notation (replaces 4234, 2234): <https://www.rfc-editor.org/rfc/rfc5234>