Added BugsnagPlugin.h to podspec

[tomlongridge]

Goal

bugsnag-cocoa v5.23.0 adds the BugsnagPlugin.h public header that needs to be included in bugsnag-react-native's podspec to allow BugsnagReactNative/Bugsnag.h to be successfully imported.

Changeset

• BugsnagReactNative.podspec - reformatted public_header_files to better match bugsnag-cocoa for ease of future updates
• CONTRIBUTING.md - added note to prompt for this to be checked in future updates.

Tests

Verified locally by creating a fresh RN project, installing bugsnag-react-native and adding #import <BugsnagReactNative/Bugsnag.h> to a source file.

Review

For the submitter, initial self-review:

• [ ] Commented on code changes inline explain the reasoning behind the approach
• [ ] Reviewed the test cases added for completeness and possible points for discussion
• [x] A changelog entry was added for the goal of this pull request
• [x] Check the scope of the changeset - is everything in the diff required for the pull request?
• This pull request is ready for:
  • [ ] Initial review of the intended approach, not yet feature complete
  • [ ] Structural review of the classes, functions, and properties modified
  • [x] Final review

For the pull request reviewer(s), this changeset has been reviewed for:

• [ ] Consistency across platforms for structures or concepts added or modified
• [ ] Consistency between the changeset and the goal stated above
• [ ] Internal consistency with the rest of the library - is there any overlap between existing interfaces and any which have been added?
• [ ] Usage friction - is the proposed change in usage cumbersome or complicated?
• [ ] Performance and complexity - are there any cases of unexpected O(n^3) when iterating, recursing, flat mapping, etc?
• [ ] Concurrency concerns - if components are accessed asynchronously, what issues will arise
• [ ] Thoroughness of added tests and any missing edge cases
• [ ] Idiomatic use of the language