Fix for iOS Cocoapods default config (#205)

Summary:
Solves #203

- [x] Sample Objc and Swift apps now have default configs(i.e non-recursive header search paths)
- [x] Fix for the cocoapods default config by adding FBMacros.h file in `Core` subspec
- [x] Updated the Spec(0.6.15) so that 0.6.15 works for default configs
- [x] Updated the doc
Pull Request resolved: https://github.com/facebook/flipper/pull/205

Reviewed By: passy

Differential Revision: D9148358

Pulled By: priteshrnandgaonkar

fbshipit-source-id: e6d32d9cde7b100573bd9a9996f0ec850b269d28

Specs/SonarKit/0.6.15/SonarKit.podspec

@@ -57,7 +57,7 @@ Pod::Spec.new do |spec|
     ss.dependency 'OpenSSL-Static', '1.0.2.c1'
     ss.compiler_flags = folly_compiler_flags
     ss.source_files = 'iOS/SonarKit/FBDefines/*.{h,cpp,m,mm}', 'iOS/SonarKit/CppBridge/*.{h,mm}', 'iOS/SonarKit/FBCxxUtils/*.{h,mm}', 'iOS/SonarKit/Utilities/**/*.{h,m}', 'iOS/SonarKit/*.{h,m,mm}'
-    ss.public_header_files = 'iOS/SonarKit/**/{SonarClient,SonarPlugin,SonarConnection,SonarResponder,SKMacros}.h'
+    ss.public_header_files = 'iOS/FBDefines/FBMacros.h', 'iOS/SonarKit/**/{SonarClient,SonarPlugin,SonarConnection,SonarResponder,SKMacros}.h'
     header_search_paths = "\"$(PODS_ROOT)/SonarKit/iOS/SonarKit\" \"$(PODS_ROOT)\"/Headers/Private/SonarKit/** \"$(PODS_ROOT)/boost-for-react-native\" \"$(PODS_ROOT)/DoubleConversion\" \"$(PODS_ROOT)/PeerTalkSonar\""
     ss.pod_target_xcconfig = { "USE_HEADERMAP" => "NO",
                              "DEFINES_MODULE" => "YES",