Update user-guide/node-selection/index.md to include pod affinity/anti-affinity

[davidopp]

and addd an example. Also clean up user-guide/node-selection/index.md a bit.

I'm not sure why the code include lines aren't working; it seems to be following the directions in
http://kubernetes.io/editdocs/
for how to include text from another file.

@kubernetes/docs to review and hopefully answer the question above

cc/ @kevin-wangzefeng @janetkuo

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This change is

[devin-donnelly]

Review status: 0 of 4 files reviewed at latest revision, 2 unresolved discussions.

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docs/user-guide/node-selection/index.md, line 99 [r1] (raw file):

### Node affinity (alpha feature)

Node affinity was introduced in Kubernetes 1.2. The design doc is [here](https://github.com/kubernetes/kubernetes/blob/{{page.githubbranch}}/docs/design/nodeaffinity.md).

I'd move the design doc link to the end of the section as a "for more information" type thing; lead with what Node Affinity actually does (third sentence).

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docs/user-guide/node-selection/index.md, line 131 [r1] (raw file):

### Inter-pod affinity and anti-affinity (alpha feature)

Inter-pod affinity and anti-affinity were introduced in Kubernetes 1.4. The design doc is [here](https://github.com/kubernetes/kubernetes/blob/{{page.githubbranch}}/docs/design/podaffinity.md).

I'd put the design doc link at the end so as not to bury the lead (i.e. what Inter-pod affinity and anti-affinity allow you to do).

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Comments from Reviewable

[devin-donnelly]

Docs LGTM pending a couple of suggestions.

[davidopp]

Updated.

[devin-donnelly]

If we can retarget this PR to the release-1.4 branch, I can merge it now. Otherwise, it'll have to wait until 1.4 goes out.

[kevin-wangzefeng]

LGTM, thanks!

[davidopp]

New commit pushed.

[davidopp]

OK I was able to resuscitate the files I accidentally deleted. PTAL.

[devin-donnelly]

Looks good. Merging now.