Build: Read nuget API key from file

After moving owner of repo, the build server was no longer authorized to push
nugets.

Update publish script to read API key from file
Ignore Build/nuget-apikey.txt
Add placeholder file with instructions

Author: angularsen

.gitignore

@@ -109,3 +109,4 @@ UpgradeLog*.XML
 
 Artifacts
 Thumbs.db
+/Build/nuget-apikey.txt

Build/nuget-apikey-placeholder.txt

@@ -0,0 +1,6 @@
+INSERT API KEY HERE
+
+* Copy this file to nuget-apikey.txt
+* Edit the new file
+* Insert API key above, as the only line in the file
+* Remove all other lines so the file only has one line with the API key

Build/nuget-publish.ps1

@@ -1,9 +1,13 @@
 $root="$PSScriptRoot\.."
 $nugetDir="$root\Artifacts\NuGet\"
+$apiKeyFile="nuget-apikey.txt"
+
+$apiKey = Get-Content $apiKeyFile
+Write-Host "Using API key: $apiKey"
 
 $list = gci -Path $nugetDir -Filter UnitsNet.*.nupkg | where-object {$_.Name -NotMatch "\.symbols\."}
 foreach ($file in $list)
 {
-  write-host "Publishing nuget: $($file.Name)"
-  Invoke-Expression "$root\Tools\nuget.exe push $($file.FullName)"
+  Write-Host "Publishing nuget: $($file.Name)"
+  Invoke-Expression "$root\Tools\nuget.exe push -ApiKey $apiKey $($file.FullName)"
 }