Add explainer for affiliations

Closes #182

Author: ehashman

index.md

@@ -16,6 +16,8 @@ We urge those in a position to do so to stop supporting the Free Software Founda
 
 To sign, please email digitalautonomy at riseup.net or [submit a pull request](https://github.com/rms-open-letter/rms-open-letter.github.io/pulls).
 
+Institutional affiliation is provided for identification purposes only and does not constitute institutional endorsement.
+
 Signed,
 
 - Molly de Blanc (Debian Project, GNOME Foundation)