Change pull request template for preview 2 branch.

Author: tkremenek

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,47 +1,13 @@
-<!-- Please complete this template before creating pull request. -->
-#### What's in this pull request?
-<!-- Description about pull request. -->
+- **Explanation**: <!-- Please provide a brief but clear explanation of the issue fixed. -->
 
-#### Resolved bug number: ([SR-](https://bugs.swift.org/browse/SR-))
-<!-- If this pull request resolves any bugs from Swift bug tracker -->
+- **Scope**: <!-- Please describe the scope of impact of the change.  -->
 
-* * * *
+- **Issue**: [SR-XXX](https://bugs.swift.org/browse/SR-XXX) and/or [rdar://problem/XXX](rdar://problem/XXX)
 
-<!-- This selection should only be completed by Swift admin -->
-Before merging this pull request to apple/swift repository:
-- [ ] Test pull request on Swift continuous integration.
+- **Risk**: <!-- The risk of taking the change, "Low", "Medium", etc., with simple rationale. -->
 
-<details>
-  <summary>Triggering Swift CI</summary>
+- **Testing**: <!-- What testing was done, added, to make sure the change is tested. -->
 
-The swift-ci is triggered by writing a comment on this PR addressed to the GitHub user @swift-ci. Different tests will run depending on the specific comment that you use. The currently available comments are:
+<!-- Once the PR is created, please assign to someone to do code review and sign off on change -->
 
-**Smoke Testing**
-
-        Platform     | Comment
-        ------------ | -------------
-        All supported platforms     | @swift-ci Please smoke test
-        All supported platforms     | @swift-ci Please smoke test and merge
-        OS X platform               | @swift-ci Please smoke test OS X platform
-        Linux platform              | @swift-ci Please smoke test Linux platform
-
-**Validation Testing**
-
-        Platform     | Comment
-        ------------ | -------------
-        All supported platforms     | @swift-ci Please test
-        All supported platforms     | @swift-ci Please test and merge
-        OS X platform               | @swift-ci Please test OS X platform
-        OS X platform               | @swift-ci Please benchmark
-        Linux platform              | @swift-ci Please test Linux platform
-
-
-**Lint Testing**
-
-        Language     | Comment
-        ------------ | -------------
-        Python       | @swift-ci Please Python lint
-
-Note: Only members of the Apple organization can trigger swift-ci.
-</details>
-<!-- Thank you for your contribution to Swift! -->
+<!-- Afterwards, assign to the release manager (tkremenek) for approval. -->