add a README

README.md

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+# Invite Links for Delta Chat
+
+This repository contains the files for <https://i.delta.chat>
+which is used to handle so called "Invite Links".
+
+"Invite Links" are an alternative
+to scan each other's QR code to get in contact or to join groups.
+
+You can share these links over any 3rd party platform.
+If recipients taps the links, they are
+either offered to install Delta Chat and tap an "Open Chat" link
+or, depending on the platform, an installed Delta Chat opens directly.
+
+The links contain the same data as Delta Chat's [OPENPPG4FPR](https://github.com/deltachat/interface/blob/master/uri-schemes.md#openpgp4fpr-) QR codes,
+but with OPENPGP4FPR scheme removed.
+
+Example:
+
+```
+https://i.delta.chat/#807051AD3E806754D35B209D788D9A0DDEAC3DB7&a=alice%40example.org&n=&i=7c5GSncKpIf&s=56k2HTPQCMd
+```
+
+The example is not functional, to create real invite links,
+open Delta Chat and select **QR code → Show QR Invite Code → Share**
+
+Note, that all data are added to the [URI Fragment](https://en.wikipedia.org/wiki/URI_fragment),
+by that, no user data is sent to the server.
+
+<https://i.delta.chat> does not get any user data that way
+and only exists to deliver some static files.
+
+
+### Developing
+
+You can clone this repository, edit the files and open `index.html` -
+as everything's static, you can test and play around.
+Even "Open Chat" should work if Delta Chat is installed.
+
+However, note, that local or other sever's links cannot open Delta Chat directly.