Add doc for source-map option to using.md.

doc/using.md

@@ -850,6 +850,33 @@ $ ./bloaty -c config.bloaty -d bloaty_package,compileunits bloaty
  100.0%  29.5Mi 100.0%  6.69Mi    TOTAL
 ```
 
+# Source maps
+
+Bloaty provides a `--source-map` option which allows
+specifying a source map for the binary. It takes an ID and
+source map file name, separated by '='. The ID can be the
+build ID or path to the binary file as specified in the
+command line.
+
+For example, to use the `compileunits` and `inlines` data
+sources for Wasm, a source map must be provided. For Wasm,
+the ID can be the text in the `sourceMappingURL` section of
+the binary.
+
+```cmdoutput
+$ ./bloaty -d compileunits --domain=file --source-map=o.wasm=o.wasm.map o.wasm
+    FILE SIZE   
+ -------------- 
+  45.0%      49    [section sourceMappingURL]
+  19.3%      21    Main.java
+  12.8%      14    [section Export]
+   7.3%       8    [WASM Header]
+   6.4%       7    [section Code]
+   5.5%       6    [section Type]
+   3.7%       4    [section Function]
+ 100.0%     109    TOTAL
+```
+
 # Source filter
 
 Sometimes, you are only interested in parts of the binary