add minimal README for 631 students

README.md

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+# sym-while, Symbolic Execution for an IMP-like Language
+
+## Building
+
+The build has three dependencies:
+
+1. You must have [OPAM](https://opam.ocaml.org/)
+2. You must have `ocamlfind` and `ocamlbuild`:
+
+    % opam install ocamlfind
+    % opam install ocamlbuild
+
+3. You must have [Z3](https://github.com/Z3Prover/z3) _with the OCaml bindings_.
+    The OCaml bindings can [only be installed from a source build](https://github.com/Z3Prover/z3/issues/1329).
+
+Make sure you have done (1) and (2) before you do (3). The source build will detect if you have OPAM/ocamlfind
+and install Z3 as an ocamlfind package if you do.
+See: [https://github.com/Z3Prover/z3/tree/master/src/api/ml](https://github.com/Z3Prover/z3/tree/master/src/api/ml).
+
+A note: follow carefully the instructions for building Z3. You must pass a specific flag (--ml) to the Python script
+in order to have the build generate and install the OCaml bindings.
+
+After you have the dependencies, you should just be able to do:
+
+    % make
+
+to build the executable.
+
+This has only been tested on OCaml 4.04, so to be safe I recommend:
+
+    % opam switch 4.04
+
+before you resolve any of the dependencies.
+
+## Examples
+
+The res/ directory has a number of examples. You can run them concretely:
+
+    % ./symwhile.native --semantics concrete res/<example_program>
+
+or symbolically:
+
+    % ./symwhile.native --semantics symbolic res/<example_program>
+