Add instruction for joining the announcement mailing list

[kara-mela]

PaNET has an announcement mailing list. Currently, this mailing list is not mentioned on the webpage, let alone how to join it.

[kara-mela]

Step by step guide

2. Go to the public page of the mailing list.
3. Enter your email address and name.
4. Press the blue button "I subscribe to list panet-announce".
5. The page updates and you need to confirm (blue button).
6. You receive a mail by SYMPA with the title "DESY Mailing lists services" and including a link. Click it.
7. A German text asks you to confirm your subscription by clicking the button "Bestätigen" while the "Abonnieren" is marked with a checkmark.
8. You receive a welcome mail by panet-announce-request@desy.de

Where should it be added to?

• README.md
• scope of homepage
• ...?