Add Debugging.md (#149)

See #148

Debugging.md

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+This document describes conventions regarding debugging.
+
+# DWARF
+
+It is a goal to support DWARF in WebAssembly, see
+[the proposed additions to DWARF](https://yurydelendik.github.io/webassembly-dwarf/)
+for how that is planned to work.
+
+## External DWARF
+
+That proposal allows keeping the DWARF information
+[external to the wasm](https://yurydelendik.github.io/webassembly-dwarf/#external-DWARF).
+When doing so, the main wasm file does not need to contain any debug info, and
+instead has a custom section with the name `external_debug_info`. That section
+contains:
+
+| Field         | Type        | Description                        |
+| ------------- | ----------- | ---------------------------------- |
+| path_name_len | `varuint32` | Length of `path_name_str` in bytes |
+| path_name_str | `bytes`     | Path to debug info file            |
+
+`path_name` is the location of a file containing DWARF debug info. Note that it
+may also contain the full wasm file as well, which can be simpler to handle
+(and tends to have little downside, as DWARF size tends to be much bigger than
+wasm size anyhow).