Fix #150 - Add "Maintaining Translations" policy to the Contributing Gui...

...delines.

@vreixo Can you please review the below guidelines and see if these make sense to you, based on your experience with translations in OTP Android so far?  I wasn't able to find a best practices for maintaining translations anywhere, so this is my current best guess at how to efficiently maintain translations.  I agree that we should minimize the effort of volunteer translation contributors, so I included your template idea as well.  If this makes sense as its currently written, I'll merge this and we can proceed with the below process.  If not, please comment on how you think it could be improved.

Also, I plan on updating the English strings today - I want to do this before requesting new translations.