Add migration instructions for Meteor 3

[perbergland]

Add a deprecation notice to the readme and instructions on how to migrate the code to Meteor 3 and its Express-based webapp

[github-actions]

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[StorytellerCZ]

Added initial deprecation notice. Will release deprecated version 1.2.0 to draw more attention to this and then as I migrate my app will add a migration guide.

[matheusccastroo]

@perbergland @StorytellerCZ Not sure I understood why it needs to be deprecated.

This package creates a router that enables to pass params through the url and have some additional fancy stuff with filters etc. Does express have a direct mapping with those features?

Taking a look here, seems that express have the req.params for the url named parameters and also the filter is actually simple to implement too. Gotcha, now makes more sense 😄