README: clarify that Go 1.16 is not yet supported (#230)

Since that version is out now, this will help avoid confusion with
newcomers. The issue is pinned now, too.

While at it, stop saying why 1.14.x is no longer supported, as it's not
a maintained version of Go at this point anymore.

For #124.

README.md

@@ -2,8 +2,8 @@
 
 	GO111MODULE=on go get mvdan.cc/garble
 
-Obfuscate Go code by wrapping the Go toolchain. Requires Go 1.15 or later, since
-Go 1.14 uses an entirely different object format.
+Obfuscate Go code by wrapping the Go toolchain. Requires Go 1.15.x; note that
+support for Go 1.16 is [a work in progress](https://github.com/burrowers/garble/issues/124).
 
 	garble build [build flags] [packages]