README: add clone command and Get-the-code section

Show the actual Gitea clone command up front, before the existing
"from a clone" instructions. Note the access prerequisites (Gitea
account + SSH key, HTTPS alternative) and that credentials come from
each developer's own tablet, not the repo.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -18,6 +18,21 @@ links. You should not need any information that isn't in this repository.
 - **Project plan & history** →
   [`doc/Chatter_implementation.md`](doc/Chatter_implementation.md)
 
+## Get the code
+
+```
+git clone git@git.andykopra.com:ack/chatter.git
+cd chatter
+```
+
+Cloning needs an account on the Gitea server at `git.andykopra.com` with read
+access to `ack/chatter`, and your SSH public key added to it (Gitea → Settings →
+SSH Keys). HTTPS also works: `git clone https://git.andykopra.com/ack/chatter.git`.
+
+The clone gives you the code only. Tablet access (root password, IPs) comes from
+*your own* tablet's Copyright-and-licenses screen, not from this repo — see
+**Quick install** step 1.
+
 ## Repository layout
 
 ```