Add PowerShell snippets for postinstall (#22)

Author: tats-u

install-poetry.py

@@ -269,7 +269,21 @@ def _get_win_folder_with_ctypes(csidl_name):
 You can execute `set -U fish_user_paths {poetry_home_bin} $fish_user_paths`
 """
 
-POST_MESSAGE_CONFIGURE_WINDOWS = """"""
+POST_MESSAGE_CONFIGURE_WINDOWS = """
+You can choose and execute one of the following commands in PowerShell:
+
+A. Append the bin directory to your user environment variable `PATH`:
+
+```
+[Environment]::SetEnvironmentVariable("Path", [Environment]::GetEnvironmentVariable("Path", "User") + ";{poetry_home_bin}", "User")
+```
+
+B. Try to append the bin directory to PATH every when you run PowerShell (>=6 recommended):
+
+```
+echo 'if (-not (Get-Command poetry -ErrorAction Ignore)) {{ $env:Path += ";{poetry_home_bin}" }}' | Out-File -Append $PROFILE
+```
+"""
 
 
 class PoetryInstallationError(RuntimeError):