Start spider monkey docs (#5968)

assets/dictionary.txt

@@ -186,3 +186,6 @@ defs
 BreakpointItem
 destructured
 getExtraProps
+SpiderMonkey
+mozconfig
+JIT

docs/spider-monkey.md

@@ -0,0 +1,67 @@
+# Spider Monkey
+
+[SpiderMonkey][sm] is Mozilla's JavaScript engine written in C and C++. The Debugger server uses its
+API to do everything from setting breakpoints to receiving pause events.
+
+[sm]: https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey
+
+## JS Shell
+
+### Install
+
+It's fairly easy to install js shell once you have the appropriate dependencies. The MDN [docs][install] have a detailed installation guide.
+
+**Quick Version:**
+
+```bash
+cd <gecko>/js/src
+autoconf-2.13
+
+# This name should end with "_OPT.OBJ" to make the version control system ignore it.
+mkdir build_OPT.OBJ
+cd build_OPT.OBJ
+../configure --enable-debug --disable-optimize
+# Use "mozmake" on Windows
+make
+
+
+# Run the shell
+./js/src/js
+```
+
+#### Pro Tips
+
+One way to speed up the builds is to add `mk_add_options MOZ_MAKE_FLAGS="-j6 -s"` to your [mozconfig][config] file.
+
+[config]: https://developer.mozilla.org/en-US/docs/Mozilla/Developer_guide/Build_Instructions/Configuring_Build_Options
+[install]: https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/Build_Documentation
+
+## JIT Tests
+
+NOTE: First you have to build the JS Shell
+
+```diff
+diff --git a/js/src/jit-test/tests/debug/wasm-11.js b/js/src/jit-test/tests/debug/wasm-11.js
+index 6c84c1c..1da17f2 100644
+--- a/js/src/jit-test/tests/debug/wasm-11.js
++++ b/js/src/jit-test/tests/debug/wasm-11.js
+@@ -11,6 +11,8 @@ g.eval(`
+     var dbg = new Debugger(parent);
+ `);
+
++console.log("YO")
++
+ var i = new WebAssembly.Instance(new WebAssembly.Module(wasmTextToBinary(`
+     (module
+         (func (export "f2")
+```
+
+```
+cd <gecko>/js/src/jit-test
+./jit_test.py --no-progress -o ../build_DBG.OBJ/dist/bin/js wasm-11
+
+Exit code: 0
+YO
+Exit code: 0
+PASSED ALL
+```