Add a note to the README regarding how to get the latest Windows 10 SDK.
Add a note to the README regarding how to get the latest Windows 10 SDK.

The latest Windows 10 update installs an incomplete version of the latest Windows 10 SDK (https://developercommunity.visualstudio.com/content/problem/41913/link-fatal-error-lnk1104-in-empty-c-project.html), which `ponyc` then tries unsuccessfully to use for linking.

kulibali/ponyc,Praetonus/ponyc,mkfifo/ponyc,mkfifo/ponyc,ponylang/ponyc,mkfifo/ponyc,boemmels/ponyc,dipinhora/ponyc,cquinn/ponyc,Theodus/ponyc,boemmels/ponyc,dipinhora/ponyc,cquinn/ponyc,boemmels/ponyc,Praetonus/ponyc,kulibali/ponyc,ponylang/ponyc,mkfifo/ponyc,Perelandric/ponyc,Perelandric/ponyc,Praetonus/ponyc,sgebbie/ponyc,boemmels/ponyc,Perelandric/ponyc,Theodus/ponyc,sgebbie/ponyc,Theodus/ponyc,Perelandric/ponyc,dipinhora/ponyc,mkfifo/ponyc,CausalityLtd/ponyc,cquinn/ponyc,ponylang/ponyc,Theodus/ponyc,sgebbie/ponyc,CausalityLtd/ponyc,sgebbie/ponyc,CausalityLtd/ponyc,Perelandric/ponyc,jemc/ponyc,cquinn/ponyc,boemmels/ponyc,kulibali/ponyc,kulibali/ponyc,jemc/ponyc,Praetonus/ponyc,Theodus/ponyc,sgebbie/ponyc,jemc/ponyc