doc/go1.16: start draft release notes

This template is based on CL 220278 and previous ones like it.
Include Compiler and Linker sections proactively, they can be
removed if they don't end up being needed for Go 1.16.

Use two spaces of indentation for TODOs to set a better precedent
for the final text that will take its place.

'relnote -html' does not report any changes at this time.

For #40700.

Change-Id: I096b0ce0d33aaaa6fae9c91c0d2dfb89b9c5e94c
Reviewed-on: https://go-review.googlesource.com/c/go/+/248198


Reviewed-by: Carlos Amedee <carlos@golang.org>