Create major_change.md

Author: oli-obk

.github/ISSUE_TEMPLATE/major_change.md

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+---
+name: Major change announcement
+about: Propose a major change.
+title: "(My major change proposal)"
+labels: major-change
+assignees: ''
+
+---
+
+# Directions
+
+If you'd like to propose a major change to do to rustc, you've come to
+the right place! To do so, please write-up your proposal in this issue.
+
+Describe your major change in a sentence or two under the TLDR section.
+
+If available, add links and more descriptions in the "Links and Details" section.
+Note: it is not expected that you write a fully fleshed out proposal there.
+Just add any information you think is important.
+
+List people already mentoring you on this change or reviewing this change in the "Mentors or Reviewers" section
+If there are none, you can also add people that you think would be a good fit.
+You can look at [the experts map](https://github.com/rust-lang/compiler-team/blob/master/content/experts/map.toml) for ideas.
+
+Oh, and please delete this section before you open the issue -- but
+keep the others!
+
+# TLDR
+
+# Links and Details
+
+# Mentors or Reviewers
+
+* Chuck Norris
+* Obi Wan Kenobi