README.md: delete outdated roadmap section
README.md: delete outdated roadmap section

Change-Id: I61201f7797e4f81080037f47f2ec8ea27453fdd4
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/309491
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite