Added CODEOWNERS and MAINTAINERS files (#240)

This PR combines all the changes from the following PRs for updating the maintainers for notaryproject/notaryproject sub-project:
Add Milind Gokarn #228
Add Pritesh Bandi #227
Add Shiwei Zhang #234
Add Toddy Mladenov #236
Add Vani Rao #229
Add Yi Zha #237

To keep a balance I dropped Add Feynman Zhou #235. The proposal is to abandon the above PRs and merge only this one.

Signed-off-by: Toddy Mladenov toddysm@gmail.com

CODEOWNERS

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+# Org-Level Owners (in alphabetical order)
+* @justincormack 
+* @niazfk
+* @stevelasker
+
+# Repo-Level Owners (in alphabetical order)
+# Note: This is only for the notaryproject/notaryproject repo
+* @gokarnm
+* @priteshbandi
+* @shizhMSFT
+* @toddysm
+* @vaninrao10
+* @yizha1

MAINTAINERS

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+# Org-Level Maintainers (in alphabetical order)
+# Pattern: [First Name] [Last Name] <[Email Address]> ([GitHub Handle])
+Justin Cormack <justin.cormack@docker.com> (@justincormack)
+Niaz Khan <niazfk@amazon.com> (@niazfk)
+Steve Lasker <StevenLasker@hotmail.com> (@stevelasker)
+
+# Repo-Level Maintainers (in alphabetical order)
+# Note: This is for the notaryproject/notaryproject repo
+Milind Gokarn <gokarnm@amazon.com> (@gokarnm)
+Pritesh Bandi <priteshbandi@gmail.com> (@priteshbandi)
+Shiwei Zhang <shizh@microsoft.com> (@shizhMSFT)
+Toddy Mladenov <toddysm@gmail.com> (@toddysm)
+Vani Rao <vaninrao@amazon.com> (@vaninrao10)
+Yi Zha <yizha1@microsoft.com> @yizha1