documentation for breakpad symbol format

[willkg]

This project should either document the breakpad symbol format and whatever additional things it's doing or link to documentation for that somewhere.

The breakpad docs have this:

https://chromium.googlesource.com/breakpad/breakpad/+/master/docs/symbol_files.md

However, that doesn't talk about the INFO lines that dump_syms adds. Are there other enhancements dump_syms does?

[willkg]

For sym files for Windows modules, it adds:

INFO CODE_ID [code_id] [code_file]

For sym files for non-Windows modules, it may add:

INFO CODE_ID [code_id]

but the code_id isn't meaningful.

dump_syms also adds a line about the generator which was added in #501 for dump_syms v2.1.0 (November 2022):

INFO GENERATOR [name] [version]