Explicitly pin the mir-json version that SAW requires

SAW's MIR backend requires a particular MIR JSON schema, but it is not entirely
obvious which version of the JSON schema to use (#2111). This patch is one step
towards addressing this concern. It:

* Adds `mir-json` as a submodule. At present, nothing in the repo (CI or
  otherwise) actually _builds_ this submodule. Its presence is purely to
  communicate which version of `mir-json` must be used to compile Rust code to
  JSON that SAW can ingest.

* Documents this in the `README`.

In the future, we will want to actually build and use `mir-json` in the CI
(see #1868 for an in-progress attempt at this), but in the meantime, this is a
decent first step. Until we actually start building `mir-json` in the CI and
using it, we will need to remember to bump the `mir-json` submodule each time
that SAW's JSON schema requirement changes.

Addresses one part of #2111.

.gitmodules

@@ -49,3 +49,6 @@
 [submodule "deps/lmdb"]
 	path = deps/lmdb
 	url = https://github.com/GaloisInc/lmdb.git
+[submodule "deps/mir-json"]
+	path = deps/mir-json
+	url = https://github.com/GaloisInc/mir-json.git

README.md

@@ -88,6 +88,18 @@ will be possible for all language constructs. There are various
 instructions that are not supported during verification. However,
 any failure during `llvm_load_module` should be considered a bug.
 
+## Notes on Rust
+
+SAW has experimental support for analyzing Rust programs. To do so, one must
+compile Rust code using [`mir-json`](https://github.com/GaloisInc/mir-json), a
+tool which compiles Rust code to a machine-readable, JSON-based format. Each
+version of SAW understands the JSON output of a particular version of
+`mir-json`, so make sure that you build the version `mir-json` that is included
+in the `mir-json` submodule (located in `deps/mir-json`).
+
+For more information on how to install `mir-json`, follow the instructions
+[here](https://github.com/GaloisInc/mir-json#installation-instructions).
+
 ## Notes on Windows
 
 If you have trouble loading the SAW REPL on Windows, try invoking it