Add readme file, rename license file.

Readme.md

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+# IMAP Cleanup
+
+IMAP Cleanup is a small .NET Core application to cleanup an IMAP inbox and leave only a number of most recent messages behind. 
+
+It uses SSL by default to connect to the IMAP server. 
+
+## Requirements
+
+You need to have the .NET Core 3.1 SDK installed to build this. You can use Visual Studio Community edition to build it.
+
+## Building
+
+There are 3 way to build the project.
+
+1. Use Visual Studio.
+2. Run the `build.cmd` script to build a Windows 64-bit self-contained executable.
+3. Use the `dotnet` tool from the command line: `dotnet build ImapCleanup.sln`
+
+## Usage
+
+Once built, you can call the executable using command line arguments to point it to your IMAP inbox, like this:
+
+	.\ImapCleanup.exe --hostname imap.mailserver.com --port 993 --username jack@foobar.com --password horsestaplebattery --count 500
+
+This will delete all messages, except the 500 most recent ones.
+
+For help, use `.\ImapCleanup.exe /?`
+
+## Disclaimer
+
+This tool deletes emails. **USE AT YOUR OWN RISK.**
+
+This little tool works fine for me. It deletes the oldest messages, leaves the right ones and that's it.
+I haven't tested this thoroughly on every possible email server and IMAP configuration in the world. So if this ends up deleting vital emails you're on your own I'm afraid.
+Before you point this to anything critical, test it on a dummy inbox with some test emails, just to be sure. 
+