Add maintainers doc

[annajung]

WG leads are listed in OWNERS files in each repo. However, there isn't one place that references all the leads.

Part of cncf/sandbox#196,

Create maintainer list + add to aggregated https://maintainers.cncf.io/ list by submitting a PR to it

This PR adds a MAINTAINERS md file that contains a reference to all the WG's OWNERS files and will be used in https://maintainers.cncf.io/

@zijianjoy @james-jwu

[annajung]

This PR would help resolve this comment cncf/foundation#662 (comment) where CNCF is looking for an aggregated list. Right now, there isn't a link to support the aggregated list of maintainers and this PR would resolve that issue

[zijianjoy]

/lgtm
/approve

Thank you Anna!

[terrytangyuan]

LGTM

[annajung]

@james-jwu can you PTAL, thanks!

[james-jwu]

/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: annajung, james-jwu, terrytangyuan, zijianjoy

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• OWNERS [james-jwu]

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