docs(labels): add pr labels to contrib (uber#264)

CONTRIBUTING.md

@@ -7,26 +7,39 @@ This document helps you navigate the process.
 
 1.  Write a mini-RFC on the component you'd like to add, and send a PR. You can find [examples here](https://github.com/uber-web/baseui/tree/master/rfcs).
 
-* It is ok, if your new component just implements the features you need, we may extend that later
+- It is ok, if your new component just implements the features you need, we may extend that later
 
 2.  Once your RFC PR is approved, start implementing the component
 
-* To make the review process fast, please try to send small PRs, if you can
-* All T0D0s in the code have to have a corresponding issue created. Refer to the created issue in the T0D0s following the format `// TOD0(#44): Something`
+- To make the review process fast, please try to send small PRs, if you can
+- All T0D0s in the code have to have a corresponding issue created. Refer to the created issue in the T0D0s following the format `// TOD0(#44): Something`
 
 3.  Once your implementation is merged, the baseui team will release it
 
 ### Definition of done
 
-* Component has any drop-in, stateful, stateless, styled (presentation) components exported
-* The component follows the default style guides of the Design team, but can be customised and fully theme-able through React Context
-* Browser accessibility support and aria attributes
-  * Rules can be found here: https://dequeuniversity.com/rules/axe/3.0/
-  * Run `yarn lint` , which has eslint-plugin-jsx-a11y running as part of it https://github.com/evcohen/eslint-plugin-jsx-a11y#supported-rules
-  * Run `yarn e2e` for e2e test, which has AxeBuilder running as part of it
-* Styletron for CSS-in-JS styling
-* Unit tests and snapshot tests with jest and enzyme
-* Flow type coverage for all relevant component code and tests
-* Documentation via Storybook: exhaustive examples (all possible props passed examples, custom styles example, custom props example, custom subcomponents, examples of precomposed and self-composed components)
-* The PR is approved by the baseui team
-* The PR is approved by the Design Team
+- Component has any drop-in, stateful, stateless, styled (presentation) components exported
+- The component follows the default style guides of the Design team, but can be customised and fully theme-able through React Context
+- Browser accessibility support and aria attributes
+  - Rules can be found here: https://dequeuniversity.com/rules/axe/3.0/
+  - Run `yarn lint` , which has eslint-plugin-jsx-a11y running as part of it https://github.com/evcohen/eslint-plugin-jsx-a11y#supported-rules
+  - Run `yarn e2e` for e2e test, which has AxeBuilder running as part of it
+- Styletron for CSS-in-JS styling
+- Unit tests and snapshot tests with jest and enzyme
+- Flow type coverage for all relevant component code and tests
+- Documentation via Storybook: exhaustive examples (all possible props passed examples, custom styles example, custom props example, custom subcomponents, examples of precomposed and self-composed components)
+- The PR is approved by the baseui team
+- The PR is approved by the Design Team
+
+## Sending Pull Requests
+
+When send a pull request, please make sure that you have one of the [following labels](https://github.com/uber-workflow/probot-app-pr-label/blob/master/index.js#L20) set:
+
+- breaking
+- feature
+- bugfix
+- docs
+- discussion
+- release
+- prerelease
+- greenkeeping