readme - permissions fix

If you try and use Notepad.exe  or Visual Studio to save the psm1 file, you will get an error about insufficient permissions even if you are logged in as Admin.
I worked around this by using ISE as Administrator

Author: zjalexander

README.md

@@ -19,6 +19,7 @@ Install-Module -Name PowerShellForGitHub
 1) Rename ApiTokensTemplate.psm1 to ApiTokens.psm1 and update value of $global:gitHubApiToken with GitHub token for your account
   * You can obtain it from https://github.com/settings/tokens). 
   * If you don't provide GitHub token, you can still use this module, but you will be limited to 60 queries per hour. 
+  * You can't achieve this with Notepad.exe due to permission restrictions. Run PowerShell ISE as Administrator instead. 
 
 2) Import module you want to use and call it's function, e.g.