Create migration guide for rules_swift 2.x

[luispadron]

rules_swift 2.x (soon to be released) will be a major version with a few breaking changes. We should document the breaking changes and offer guidance on going from 1.x to 2.x for users and rule authors.

[luispadron]

Some things I can think of we should go over:

• Removal of swift.emit_symbol_graph feature, we should point users to the new swift_symbol_graph_extract rule.
• Changes to C/C++/Objective-C module name determination, we should point users to the new swift_interop_hint rule and use of aspect_hints.

[brentleyjones]

Removal of the deprecated grpc/proto rules.

[brentleyjones]

I'm going to cherry-pick 57cdd76, so we should tell people to use the new load statements (though not a breaking change yet, the old way will only be deprecated for now).

[aaronsky]

Removal of swift_c_module as the rule is redundant with swift_interop_hint's featureset

[brentleyjones]

swift.disable_system_index is now enabled by default

[brentleyjones]

Removed swift.bundled_xctests and swift.use_response_files.