Switch contributor instructions to use github draft mode for PRs (#618)

instead of WIP title

CONTRIBUTING.md

@@ -8,9 +8,9 @@ We welcome contributions from anyone on the internet and are grateful for even t
 2.  Clone your fork
 3.  Follow the [installation & build steps](https://github.com/0xProject/0x-tools#install-dependencies) in the repo's top-level README.
 4.  Setup the recommended [Development Tooling](#development-tooling).
-5.  Open a PR with the `[WIP]` flag against the `development` branch and describe the change you are intending to undertake in the PR description. (see [our branch naming conventions](#branch-structure))
+5.  Open a [draft PR](https://github.blog/2019-02-14-introducing-draft-pull-requests/) against the `development` branch and describe the change you are intending to undertake in the PR description. (see [our branch naming conventions](#branch-structure))
 
-Before removing the `[WIP]` tag and submitting the PR for review, make sure:
+Before making the PR "Ready for review", make sure:
 
 -   It passes our linter checks (`yarn lint`)
 -   It is properly formatted with Prettier (`yarn prettier`)

PULL_REQUEST_TEMPLATE.md

@@ -20,7 +20,6 @@
 
 <!--- The following points should be used to indicate the progress of your PR.  Put an `x` in all the boxes that apply right now, and come back over time and check them off as you make progress.  If you're unsure about any of these, don't hesitate to ask. We're here to help! -->
 
--   [ ] Prefix PR title with `[WIP]` if necessary.
 -   [ ] Add tests to cover changes as needed.
 -   [ ] Update documentation as needed.
 -   [ ] Add new entries to the relevant CHANGELOG.jsons.