[Docs] NS_ENUM exported to JS, used as method arguments for native modules, must use RCT_ENUM_CONVERTER in RCTConvert class extension

[MattFoley]

I think this section of the docs needs to be further expanded to explain the use of RCT_ENUM_CONVERTER.

In v0.5.0, I was able to export NS_ENUM defined constants, and use them as method arguments. Example code below.

Enum Definition:

    typedef NS_ENUM(NSInteger, MyEnumType)
    {
        kEnumOne,
        kEnumTwo,
        kEnumThree,
    };

Constants:

    - (NSDictionary *)constantsToExport
    {
        return @{@"OnError": @{ @"enumOne" : @(kEnumOne),
                                @"enumTwo"  : @(kEnumTwo),
                                @"enumThree"   : @(kEnumThree)}
                      }
    };

Method Definition:

RCT_EXPORT_METHOD(doSomethingWithEnum:(MyEnumType)enum
                           completion:(RCTResponseSenderBlock)callback)

In v0.7.0, this failed with the following log:

Unknown argument type 'MyEnumType' in method -[MyNativeModule
doSomethingWithEnum:completion:]. Extend RCTConvert to support this type.

I was only able to puzzle the solution out after reading through the internal React Native code for quite some time. (Create a class extension for RCTConvert, use RCT_ENUM_CONVERTER to implement a converter method.)

    @implementation RCTConvert (MyEnumExtension)
    RCT_ENUM_CONVERTER(MyEnumType, (@{ @"enumOne" : @(kEnumOne),
                                       @"enumTwo"  : @(kEnumTwo),
                                       @"enumThree"   : @(kEnumThree)), 
                       kEnumOne, integerValue)
    @end

[brentvatne]

@MattFoley - thanks for the detailed description! If you can write up the docs for this and submit a PR I'll be happy to review, just ping me on it!

[MattFoley]

Will do, thanks @brentvatne.