appveyor: Only build on changes to certain files
appveyor: Only build on changes to certain files

Primarily we want to not build when README files, images
and other files that are not used by the build are changed.

Build on changes to these:
.sln
.csproj
.cs
.json
.ps1
.psm1
.bat

anjdreas/UnitsNet,anjdreas/UnitsNet