Add ziarno:restrict-mixin and ziarno:provide-mixin to readme

[ziarno]

• [ ]  General Comment
• <img border=0 src='https://avatars.githubusercontent.com/u/4250050?v=3' height=16 width=16'> @ziarno7: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Meteor Contributor Agreement here: https://contribute.meteor.com/
• <img border=0 src='https://avatars.githubusercontent.com/u/3158482?v=3' height=16 width=16'> :+1:
• <img border=0 src='https://avatars.githubusercontent.com/u/3158482?v=3' height=16 width=16'> :shipit:
• <img border=0 src='https://avatars.githubusercontent.com/u/448783?v=3' height=16 width=16'> @ziarno7 can you sign the contributor agreement, please? This is just to verify that your work is your own.

   

[apollo-cla]

@ziarno7: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Meteor Contributor Agreement here: https://contribute.meteor.com/

[ziarno]

:+1:

[ziarno]

:shipit:

[stubailo]

@ziarno7 can you sign the contributor agreement, please? This is just to verify that your work is your own.

[stubailo]

Super belated merge! Thanks.