Description

Request from Nuget team to add a better readme to the nuget package so it is displayed nicely on nuget.org.

Previously we were using the ORT repo readme.md but that a) doesn't display correctly due to limited markdown support on nuget.org, and b) has a lot of irrelevant info like build pipeline status.

• Created a generic readme.md that includes the ORT description from the main readme, includes the ORT logo via an acceptable link, and lists the native nuget packages so the file can be included in any of them as-is.
• Updated the nuget packaging script to add thereadmetag and use this file.

Motivation and Context

Request from MS Nuget team to MS package owners to add.