Add the podspec and fix readme for React Native 0.60.0

[iamacup]

Hi

There are some updates needed to this repo to make it auto link with the new React Native 0.60.x

This pull will fix auto linking and make this error stop appearing:

'. No podspec file was found.

    - Check to see if there is an updated version that contains the necessary podspec file
    - Contact the library maintainers or send them a PR to add a podspec. The react-native-webview podspec
    is a good example of a package.json driven podspec. See
    https://github.com/react-native-community/react-native-webview/blob/master/react-native-webview.podspec
    - If necessary, you can disable autolinking for the dependency and link it manually. See
    https://github.com/react-native-community/cli/blob/master/docs/autolinking.md#how-can-i-disable-autolinking-for-unsupported-library

EDIT:

I have published my pull request on npm as react-native-screenshot-detect for easy access to this because its been annoying me for awhile now... :)

[nunomago]

Also trying to upgrade to RN 0.60 and stumbled upon this PR :) Kudos! 👍

[iamacup]

I have published my pull request on npm as react-native-screenshot-detect for easy access to this because its been annoying me for awhile now... :)