Compiler rewrite document added for collaboration

[troysgit]

Re. Zulip topic - https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Info.20re.2E.20compiler.20rewrite.20idea.20.28ncameron.2C.202023.29/near/481236544

Description

I am opening this PR to try to establish a central document for experts in the Rust compiler community to write their thoughts about a potential rewrite of the compiler.

Changes Made

1. Document 'compiler-rewrite.md' added as an appendix to serve as a central document for authors to share their opinions and can then be synthesised into one piece of work as opposed to multiple separate blog posts or similar dispersed across the net.
2. A reference to 'compiler-rewrite.md' added in 'SUMMARY.md'.

Additional Notes

This will undoubtedly require changes in formatting, location, etc. but I hope it serves as a starting point for contributors to use.

[oli-obk]

I don't think this is the right way to go about this if at all. No one had interest in the zulip thread, so I don't think anyone will be adding to your PR either.