Add testing/modifying instructions to README (#36)

@bsmth As discussed in
#35 (comment)

This adds links to WABT, the docs on MDN, and to the table for supported
features.

---------

Co-authored-by: Brian Thomas Smith <brian@smith.berlin>

README.md

@@ -1,2 +1,18 @@
 # webassembly-examples
-Code examples that accompany the MDN WebAssembly documentation — see https://developer.mozilla.org/en-US/docs/WebAssembly. 
+
+Code examples that accompany the MDN WebAssembly documentation — see https://developer.mozilla.org/en-US/docs/WebAssembly.
+
+## Testing/modifying the examples
+
+The examples can be tested locally by running a [local server](https://developer.mozilla.org/en-US/docs/Learn/Common_questions/Tools_and_setup/set_up_a_local_testing_server#using_python) to serve your directory of choice.
+
+If you modify any `.wat` files for testing you will need to generate a corresponding `.wasm` file, replacing the existing version in the folder.
+This can be done using the `wat2wasm` tool, which is part of the [WABT: The WebAssembly Binary Toolkit](https://github.com/WebAssembly/wabt/) (for setup/usage see [Converting the text .wat into a binary .wasm file](https://developer.mozilla.org/en-US/docs/WebAssembly/Text_format_to_Wasm#converting_the_text_.wat_into_a_binary_.wasm_file) on MDN and the readme in the WABT GitHub repo.
+
+Note that some examples use features that are still considered optional.
+These are listed in the [supported proposals](https://github.com/WebAssembly/wabt/#supported-proposals) section on the WABT README.md, along with the flags used to invoke them.
+For example, to build WASM for the [multi-memory](https://github.com/mdn/webassembly-examples/blob/main/understanding-text-format/multi-memory.wat) example you will need to specify the `--enable-multi-memory` flag as shown:
+
+```sh
+wat2wasm --enable-multi-memory multi-memory.wat
+```