docs: add note to README about Ionic and Cordova using InAppBrowser plugin for auth

[jeffbcross]

Auth APIs require location.protocol to be http or https, whereas in Ionic and Cordova the protocol is file.

Related issues: #279, #287

[jeffbcross]

I'm not really sure about this one, since I'm not set up to verify that it's correct. Going to unassign it from myself since I'll be out a few days.

Here's what I'd started adding to Readme:

Note for Ionic and Cordova Users

Certain Firebase auth APIs, such as signInWithPopup, require the
value of location.protocol to be http or https, which is
different from the default protocol of file inside Cordova apps.
To work around this issue, please use the InAppBrowser Cordova
plugin. Ionic users can install this plugin like so:

$ ionic plugin add cordova-plugin-inappbrowser

[Ronsku]

I have the same problem even with the "inappbrowser"-plugin installed :(.

[bnova-stefan]

I have also this problem even with inappbrowser plugin installed

[samirvarandas]

Me too.

[jayxC]

Any fix for this issue?

[mubasshir]

not working with InAppBrowser too
@jeffbcross any working snippet?

[randbrown]

Any fix for this yet?

[davideast]

@randbrown This is an issue that InAppBrowser doesn't fix unfortunately.

You may be able to use a login library and take the resulting token to login. We're going to look at using libraries like this: https://github.com/fuffenz/ionic2-native-facebook-login

But we're focused on other issues atm. If you'd like to a take a look at let us know what you've encountered that would be great!