Elaborate on check list items in PR template (#101)

The pull request template has a number of items contributors can
check to classify their changes. This commit updates the template
to give more details about the types of changes that impact the
checkboxes along with some examples.

Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>

Author: makubacki

.sync/github_templates/pull_requests/pull_request_template.md

@@ -12,10 +12,26 @@ For each item, place an "x" in between `[` and `]` if true. Example: `[x]`.
 _(you can also check items in the GitHub UI)_
 
 - [ ] Impacts functionality?
+  - **Functionality** - Does the change ultimately impact how firmware functions?
+  - Examples: Add a new library, publish a new PPI, update an algorithm, ...
 - [ ] Impacts security?
+  - **Security** - Does the change have a direct security impact on an application,
+    flow, or firmware?
+  - Examples: Crypto algorithm change, buffer overflow fix, parameter
+    validation improvement, ...
 - [ ] Breaking change?
+  - **Breaking change** - Will anyone consuming this change experience a break
+    in build or boot behavior?
+  - Examples: Add a new library class, move a module to a different repo, call
+    a function in a new library class in a pre-existing module, ...
 - [ ] Includes tests?
+  - **Tests** - Does the change include any explicit test code?
+  - Examples: Unit tests, integration tests, robot tests, ...
 - [ ] Includes documentation?
+  - **Documentation** - Does the change contain explicit documentation additions
+    outside direct code modifications (and comments)?
+  - Examples: Update readme file, add feature readme file, link to documentation
+    on an a separate Web page, ...
 
 ## How This Was Tested