MSTG-CODE-1 Enhance iOS Code Signing Section (by @NowSecure)

[cpholguera]

Add more details and references.

[vadimszzz]

@cpholguera Take a look at:
https://github.com/vadimszzz/iOS-Internals-and-Security-Testing#application-security-features-apple-fairplay-drm
https://github.com/vadimszzz/iOS-Internals-and-Security-Testing#aes-keys
https://github.com/vadimszzz/iOS-Internals-and-Security-Testing#get-decrypted-ipa-file
https://redmaple.tech/blogs/macho-files/#code-signing-segment
https://github.com/vadimszzz/codesign/blob/master/codesign.py#L15
https://www.first.org/resources/papers/conf2016/FIRST-2016-130.pdf - page 26

I can contribute to OWASP MSTG with all the following information about code signing. And much more in other topics.

[cpholguera]

Hi @vadimszzz, that's a great idea! We'd love it if you start contributing. I'm sure we can find many topics for you, starting with this one for example. Let me merge this PR and if you'd like to send a new one with your improvements for the code signing section please go ahead.

Regarding other topics let's make a list and I'll open several issues and assign them to you. Are you in our Slack? I'd recommend that you join it so that we can chat over there.

https://github.com/OWASP/owasp-mstg#connect-with-us

I'll officially invite you to the project as a collaborator so that I can assign you issues. You should receive a notification/email from GitHub soon.