Update Readme for iOS manual install

[markrickert]

Add manual installation instructions for iOS as well as make sure that our react-native link command specifies the project we want to link.

[tristandl]

thanks, these comments really helped. i have a couple more suggestions:

• make it clear that you can use npm install instead of yarn if you are already using npm
• point out that in xcode the .a file won't be found (and is in the Products subdir) so it will be red, but you can still drag it in to be a linked framework
• make it clear that if you do the manual steps (we had to) you don't need to do react-native link (i think..)
• point out that if you do the manual steps, you need to remove the changes made to your Podfile to avoid conflicts

I think these are the steps we followed, along with a lot of trial and error. Just another (wasted) day in the life of react-native!

[a-morenovalls]

@tristandl changes are fine, except for npm (I think most companies are using yarn instead due to performance). Please @markrickert , can you update the Readme? Thanks!

[1uss1]

@alvaromb I think this pull request should be merged as is. At the moment the master branch README.md does not contain any installation instructions for iOS. The only way I figured how to get react-native-version-number working on iOS was by finding this pull request.

[markrickert]

Some info is better than no info on the readme. yarn vs npm shouldn't be a blocker imho.

This is all made obsolete by react native 0.60 anyways 🤷‍♂️

[rsmelo92]

I think this could be helpful

[Tamenze]

Could a note about running cd ios && pod install be added to the Readme as well? I linked and ran without installing pods and got all null values for Version Number, until I found clues in the Issues.