Add example for encoding/writing ASAR files

[Happy-Ferret]

Based on the given documentation/tests, I struggle implementing a way to encode and write the contents of a directory to an ASAR file.

Would it be possible to extend the documentation, as to make it more clear how to achieve writing ASAR files?

[Happy-Ferret]

NVM. I figured it out.

Alas, the output is broken (due to #2) without using the patched version by yyq2013.

I'll gladly provide a PR for the example, provided the above bug is fixed.

[jaygooby]

@Happy-Ferret I implemented a pack option in cmd/asar/main.go that also uses the header fixes from https://github.com/yyq2013/asar