Update default .gitignore to work with fastlane setups in sub-folders
Update default .gitignore to work with fastlane setups in sub-folders

Summary:
Currently the default way to setup _fastlane_ for ReactNative projects is to set it up in the `ios` or `android` subfolder. This PR updates the path and also the URL to the new fastlane docs page.
Closes https://github.com/facebook/react-native/pull/13261

Differential Revision: D5567604

Pulled By: hramos

fbshipit-source-id: 89c27328bb2748ff1772812786e2821963dc1779

facebook/react-native,myntra/react-native,Bhullnatik/react-native,makadaw/react-native,myntra/react-native,farazs/react-native,rickbeerendonk/react-native,kesha-antonov/react-native,dikaiosune/react-native,hammerandchisel/react-native,frantic/react-native,exponentjs/react-native,rickbeerendonk/react-native,kesha-antonov/react-native,ptmt/react-native-macos,negativetwelve/react-native,cdlewis/react-native,exponentjs/react-native,hoastoolshop/react-native,Bhullnatik/react-native,hoastoolshop/react-native,ptmt/react-native-macos,Bhullnatik/react-native,pandiaraj44/react-native,makadaw/react-native,javache/react-native,makadaw/react-native,myntra/react-native,rickbeerendonk/react-native,cdlewis/react-native,hammerandchisel/react-native,hoastoolshop/react-native,pandiaraj44/react-native,hammerandchisel/react-native,javache/react-native,rickbeerendonk/react-native,janicduplessis/react-native,hoangpham95/react-native,janicduplessis/react-native,dikaiosune/react-native,frantic/react-native,farazs/react-native,cdlewis/react-native,exponentjs/react-native,arthuralee/react-native,hoastoolshop/react-native,hammerandchisel/react-native,exponent/react-native,jevakallio/react-native,dikaiosune/react-native,facebook/react-native,hoangpham95/react-native,rickbeerendonk/react-native,kesha-antonov/react-native,exponent/react-native,makadaw/react-native,negativetwelve/react-native,ptmt/react-native-macos,cdlewis/react-native,pandiaraj44/react-native,exponentjs/react-native,exponentjs/react-native,jevakallio/react-native,hoangpham95/react-native,dikaiosune/react-native,frantic/react-native,Bhullnatik/react-native,facebook/react-native,exponentjs/react-native,jevakallio/react-native,farazs/react-native,jevakallio/react-native,hoangpham95/react-native,myntra/react-native,jevakallio/react-native,jevakallio/react-native,farazs/react-native,hoangpham95/react-native,negativetwelve/react-native,dikaiosune/react-native,pandiaraj44/react-native,janicduplessis/react-native,dikaiosune/react-native,hoastoolshop/react-native,Bhullnatik/react-native,javache/react-native,farazs/react-native,hammerandchisel/react-native,arthuralee/react-native,facebook/react-native,facebook/react-native,javache/react-native,cdlewis/react-native,facebook/react-native,kesha-antonov/react-native,rickbeerendonk/react-native,jevakallio/react-native,farazs/react-native,pandiaraj44/react-native,dikaiosune/react-native,negativetwelve/react-native,janicduplessis/react-native,negativetwelve/react-native,facebook/react-native,hoangpham95/react-native,ptmt/react-native-macos,exponentjs/react-native,myntra/react-native,javache/react-native,frantic/react-native,frantic/react-native,makadaw/react-native,makadaw/react-native,kesha-antonov/react-native,makadaw/react-native,arthuralee/react-native,rickbeerendonk/react-native,pandiaraj44/react-native,javache/react-native,exponent/react-native,janicduplessis/react-native,janicduplessis/react-native,javache/react-native,javache/react-native,hammerandchisel/react-native,kesha-antonov/react-native,cdlewis/react-native,facebook/react-native,hammerandchisel/react-native,hammerandchisel/react-native,exponent/react-native,negativetwelve/react-native,exponentjs/react-native,makadaw/react-native,jevakallio/react-native,Bhullnatik/react-native,ptmt/react-native-macos,javache/react-native,Bhullnatik/react-native,ptmt/react-native-macos,hoastoolshop/react-native,cdlewis/react-native,kesha-antonov/react-native,rickbeerendonk/react-native,dikaiosune/react-native,janicduplessis/react-native,negativetwelve/react-native,hoastoolshop/react-native,myntra/react-native,kesha-antonov/react-native,negativetwelve/react-native,exponent/react-native,exponent/react-native,pandiaraj44/react-native,Bhullnatik/react-native,farazs/react-native,ptmt/react-native-macos,myntra/react-native,jevakallio/react-native,hoangpham95/react-native,kesha-antonov/react-native,myntra/react-native,farazs/react-native,janicduplessis/react-native,myntra/react-native,facebook/react-native,cdlewis/react-native,farazs/react-native,arthuralee/react-native,arthuralee/react-native,negativetwelve/react-native,hoangpham95/react-native,hoastoolshop/react-native,cdlewis/react-native,makadaw/react-native,ptmt/react-native-macos,exponent/react-native,rickbeerendonk/react-native,exponent/react-native,pandiaraj44/react-native