Add grammar of language to repo

Preliminary, work in progress

Grammar

@@ -0,0 +1,21 @@
+
+
+<program> := <statements> EOF
+
+<statements> := <statement> 
+             |  <statement> SEP <statements>
+
+<statement> := let <id> = <expr>
+            |  <expr>
+
+<expr> := if <expr> then <statements> end
+       |  if <expr> then <statements> else <statements> end
+       |  while <expr> SEP <statements> end
+       |  <binexpr>
+       |  <number>
+       |  <id>
+
+
+<binexpr> :=
+
+