Add README

Signed-off-by: Sylvain Leclerc <[email protected]>

grammar/README.md

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+# Expression grammar definition
+
+[Expr.g4](Expr.g4) defines the grammar for mathematical expressions
+use for defining constraints, objective, etc.
+
+[ANTLR](https://www.antlr.org) needs to be used to generate the associated
+parser code, which must be placed in [andromede.expression.parsing.antlr](/src/andromede/expression/parsing/antlr)
+package.
+
+ANTLR may be used on the command line, or for example through
+the associated PyCharm plugin.
+
+We use the visitor and not the listener in order to translate ANTLR AST
+into our own AST, so the options `-visitor` and `-no-listener` need to
+to be used.