Add extra installation instructions about administrator privileges and PSCore6 modules

从安装说明以及涵盖 Start-PSBuild、PowerShell 代码片段和 Invoke-Build 的 README 部分开始。记录需要管理员会话以及 PSCore6 模块还原步骤，然后统一命令示例；当已涵盖报告的设置问题和对提示符格式的困惑时，即视为完成。

I was trying to build PowerShellEditorServices on my machine with the following $PSVersionTable:

Name                           Value
----                           -----
PSVersion                      6.0.1
PSEdition                      Core
GitCommitId                    v6.0.1-235-g3f7494e5435bd776ad04ef426c00874410924486
OS                             Microsoft Windows 10.0.16299
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

Install-Module not found

Due to self-hosting on PSCore6, there was no Install-Module cmdlet. It might be worth having a note somewhere (possibly here or in the PowerShell README?) to make sure people use Start-PSBuild -PSModuleRestore when building PSCore6 and get the PowerShellGet module.

Invoke-Build: IsBypassed not supported error

Then at the step where Invoke-Build is used, I got the following error:

PS> Invoke-Build Build
Build Build C:\Users\roholt\Documents\PowerShellEditorServices\PowerShellEditorServices.build.ps1
Task /Build/SetupDotNet

### Installing .NET CLI 2.0.0...

C:
dotnet-install: Downloading link: https://dotnetcli.azureedge.net/dotnet/Sdk/2.0.0/dotnet-sdk-2.0.0-win-x64.zip
dotnet-install: Cannot download: https://dotnetcli.azureedge.net/dotnet/Sdk/2.0.0/dotnet-sdk-2.0.0-win-x64.zip
dotnet-install: Downloading legacy link: https://dotnetcli.azureedge.net/dotnet/Sdk/2.0.0/dotnet-dev-win-x64.2.0.0.zip
ERROR: Exception calling "IsBypassed" with "1" argument(s): "Operation is not supported on this platform."
At C:\Users\roholt\AppData\Local\Temp\dotnet-install.ps1:175 char:20
+ ...          if($DefaultProxy -and (-not $DefaultProxy.IsBypassed($Uri))) ...
+                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
At C:\Users\roholt\Documents\PowerShellEditorServices\PowerShellEditorServices.build.ps1:21 char:1
+ task SetupDotNet -Before Clean, Build, TestHost, TestServer, TestProt ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
At C:\Users\roholt\Documents\PowerShellEditorServices\PowerShellEditorServices.build.ps1:129 char:1
+ task Build {
+ ~~~~~~~~~~~~
Build FAILED. 2 tasks, 1 errors, 0 warnings 00:00:04.8974401
Exception calling "IsBypassed" with "1" argument(s): "Operation is not supported on this platform."
At C:\Users\roholt\AppData\Local\Temp\dotnet-install.ps1:175 char:20
+ ...          if($DefaultProxy -and (-not $DefaultProxy.IsBypassed($Uri))) ...
+                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo          : NotSpecified: (:) [], ParentContainsErrorRecordException
+ FullyQualifiedErrorId : PlatformNotSupportedException

I needed some trial and error to work out that I just needed to be running an administrator session. For the sake of completeness, there is a fixed dotnet-cli issue that looks like this, but we only need administrator privileges here.

PowerShell snippets in README

Also, the PowerShell snippets in the instructions are inconsistent about giving a prompt and we should make them agree. Not including a prompt makes copy-pasting easier, but including a prompt gives more information on where the user should be executing commands. The ideal is including the prompt and having it unselectable -- GitHub does this in their SSH key pages using ::before in CSS, but that can only be set once. It may not be possible in GitHub Markdown, but I've put an attempt below that doesn't work in preview, but may be a start.

 PS C:\path\to\dir> Invoke-Command "do something"

Filing this as an issue chiefly to document it for anyone else who encounters problems but also for discussion.