Update branch guidance

- Update the guidance for branches to just target the default branch.
- Update various Markdown lint errors.
- Remove Dylan's Twitter contact details until he's available again.

.github/pull_request_template.md

@@ -2,13 +2,13 @@
 
 ### The issue or feature being addressed
 
-<!-- Please include the existing github issue number where relevant -->
+<!-- Please include the existing GitHub issue number where relevant -->
 
 ### Details on the issue fix or feature implementation
 
 ### Confirm the following
 
-- [ ]  I started this PR by branching from the head of the latest dev vX.Y branch, or I have rebased on the latest dev vX.Y branch, or I have merged the latest changes from the dev vX.Y branch
-- [ ]  I have targeted the PR to merge into the latest dev vX.Y branch as the base branch
+- [ ]  I started this PR by branching from the head of the default branch
+- [ ]  I have targeted the PR to merge into the default branch
 - [ ]  I have included unit tests for the issue/feature
 - [ ]  I have successfully run a local build

CONTRIBUTING.md

@@ -1,3 +1,3 @@
-Please see our Instructions for Contributing in the [ReadMe](https://github.com/App-vNext/Polly#instructions-for-contributing) and [wiki](https://github.com/App-vNext/Polly/wiki/Git-Workflow).
+Please see our Instructions for Contributing in the [README](https://github.com/App-vNext/Polly#instructions-for-contributing) and [wiki](https://github.com/App-vNext/Polly/wiki/Git-Workflow).
 
-Please be sure to branch from the head of the latest vX.Y.Z dev branch (rather than master) when developing contributions.  
+Please be sure to branch from the head of the default branch when developing contributions.