guru: Add a TODO list to the guru cmd.

Adds a TODO list for the guru cmd. The main goal is document know
issues that we have not addressed.

Updates golang/go#47326
Updates golang/go#52503

Change-Id: I126b0f06081b606124d89a13f8805fa1ac6e56e3
Reviewed-on: https://go-review.googlesource.com/c/tools/+/403354
TryBot-Result: Gopher Robot <gobot@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>

cmd/guru/TODO

@@ -0,0 +1,11 @@
+-*- text -*-
+
+Guru to-do list
+===========================
+
+Generics:
+- decide on whether to support generics in guru
+- decide on whether to instantiate generics in ssa (go.dev/issue/52503)
+
+MISC:
+- test support for *ssa.SliceToArrayPointer instructions (go.dev/issue/47326)