Add a section on sourceMappingURL (#151)

A follow-up to #149. See #148.

Debugging.md

@@ -24,3 +24,22 @@ a Wasm container, which includes DWARF in Wasm custom sections, in the same
 format as they would appear normally in a Wasm file. Note that the container may
 also contain other sections, such as the code and data sections in the original
 Wasm file.
+
+# Source maps
+
+Adoption of DWARF is a fairly recent addition to WebAssembly, and many toolchains still
+support [Source Map](https://sourcemaps.info/spec.html) based debugging as well (or instead).
+
+Source map is an external JSON file that specifies how to map a (zero-based) line and column
+position in generated code to a file, line, and column location in the source. For
+WebAssembly binary locations, the line number is always 1, and the column number
+is interpreted as a byte offset into the WebAssembly binary content.
+Source locations are interpreted as in the source map spec.
+
+On the WebAssembly side, the URL of such Source Map file is stored in a custom section with the name
+`sourceMappingURL`. That section contains:
+
+| Field        | Type        | Description                       |
+| ------------ | ----------- | --------------------------------- |
+| url_name_len | `varuint32` | Length of `url_name_str` in bytes |
+| url_name_str | `bytes`     | URL to the Source Map file        |