Add PowerShell script for Windows installation

[Anshulgada]

This PR adds a PowerShell script (install-sendme.ps1) to allow Windows users to easily download and install the sendme tool, as well as add it to their PATH automatically.

Changes:

• Added install-sendme.ps1 to the root directory for Windows installation.
• Users can now install sendme by running a simple PowerShell command.
• Edited the README to differentiate between download method for Linux/MacOS & WIndows

Notes:

• This update is specifically for Windows users.
• The script adds the sendme folder to the system PATH, which will allow running sendme from any terminal without specifying its full path.

Testing:
Tested the script on a Windows machine with the following outcomes:

• Successfully downloaded and extracted the sendme executable.
• Verified that the folder was added to the system PATH.
• Confirmed that running the sendme command from anywhere worked as expected.

New Installation Command (Once merged):

iex ((New-Object System.Net.WebClient).DownloadString('https://raw.githubusercontent.com/n0-computer/sendme/main/install-sendme.ps1'))

[Arqu]

Thanks for the PR!
Moved the script here n0-computer/iroh.computer#309

[Anshulgada]

Hey! I've made the changes to both the script and the README.