Add google-services.json instructions to README

[ktprograms]

PR Checklist

Please check all that apply to this PR using "x":

• [x] I have checked that this PR is not a duplicate of an existing PR (open, closed or merged)
• [x] I have checked that this PR does not introduce a breaking change

PR Type

What kind of change does this PR introduce?

• [x] Documentation changes

Fixes

Issue Number: N/A