[w3ctag/design-reviews] README: fix links with spaces to use encoded spaces (#175)

Github's markdown flavour doesn't seem to support
links with spaces in them so instead use
%20 the URI encoding of a space as a substitute.

### Exhibit:

#### Before
<img width="986" alt="screen shot 2017-05-03 at 4 28 23 pm" src="https://cloud.githubusercontent.com/assets/4898263/25684262/9ac6e334-301d-11e7-97b2-4c4cb2598aee.png">


#### After
<img width="979" alt="screen shot 2017-05-03 at 4 28 07 pm" src="https://cloud.githubusercontent.com/assets/4898263/25684270/9f38bf28-301d-11e7-8790-c2956d203ddb.png">

You can view, comment on, or merge this pull request online at:

  https://github.com/w3ctag/design-reviews/pull/175

-- Commit Summary --

  * README: fix links with spaces to use encoded spaces

-- File Changes --

    M README.md (6)

-- Patch Links --

https://github.com/w3ctag/design-reviews/pull/175.patch
https://github.com/w3ctag/design-reviews/pull/175.diff

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