docs(readme): update stuff for Q2 2018

[donroyco]

Bug, feature request, or proposal:

Proposal

What is the expected behavior?

Up to date content in readme file.

What is the current behavior?

The current README.md on master has the 'High level stuff planned for Q1 2018 (January - March)' and should be updated to Quarter 2 of 2018.

What are the steps to reproduce?

Just visit the https://github.com/angular/material2 and read the Project status.

What is the use-case or motivation for changing an existing behavior?

Since version 6 of Material2 is just around the corner with a lot of great stuff, it's good to keep people up to date with the plan(s) of adding or updating features. Related issue: #9192 (for Q1 2018).

[jelbourn]

Those quarter boundaries always sneak by me.

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