docs: document the rsync-latest snapshot PPA

Add the new ppa:rsyncproject/rsync-latest (development snapshots rebuilt
from git master) alongside the existing stable PPA in INSTALL.md and the
download page.  Notes that snapshot versions (3.5.0~git...) sort below the
matching stable release, so the two PPAs can coexist without a stable
release being silently replaced by a snapshot.

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

Author: tridge

INSTALL.md

@@ -21,6 +21,18 @@ See [the PPA page][ppa] for current build status across architectures.
 
 [ppa]: https://launchpad.net/~rsyncproject/+archive/ubuntu/rsync
 
+To test the upcoming release instead, there is also a [`rsync-latest`
+PPA][ppa-latest] that is rebuilt from the tip of the git master branch.  These
+are development snapshots whose version numbers (such as
+`3.5.0~git20260601...`) deliberately sort below the matching stable release, so
+the stable PPA above will never silently move you from a release onto a
+snapshot.  Use it for testing only -- it may contain unreleased changes:
+
+>     sudo add-apt-repository ppa:rsyncproject/rsync-latest
+>     sudo apt update && sudo apt install rsync
+
+[ppa-latest]: https://launchpad.net/~rsyncproject/+archive/ubuntu/rsync-latest
+
 The rest of this document covers building from source.
 
 ## The basic setup

rsync-web/download.html

@@ -75,6 +75,20 @@ <h3>Ubuntu</h3>
 <p>See the <a href="https://launchpad.net/~rsyncproject/+archive/ubuntu/rsync">PPA page on Launchpad</a>
 for build status across architectures.
 
+<p>For testing the upcoming release, the project also maintains a
+<a href="https://launchpad.net/~rsyncproject/+archive/ubuntu/rsync-latest">rsync-latest PPA</a>
+that is rebuilt from the tip of the <a href="https://github.com/RsyncProject/rsync">git
+master branch</a>.  These are development snapshots: their version numbers
+(such as <code>3.5.0~git20260601...</code>) deliberately sort <em>below</em>
+the matching stable release, so mixing this with the stable PPA above will
+never silently move you from a release onto a snapshot:
+
+<p><code><small>sudo add-apt-repository ppa:rsyncproject/rsync-latest<br>
+sudo apt update &amp;&amp; sudo apt install rsync</small></code>
+
+<p>Use the stable PPA for production systems; the rsync-latest PPA is for
+testing the master branch and may contain unreleased changes.
+
 <p>The <a href="https://github.com/RsyncProject/rsync/actions">GitHub Actions
 page</a> has build events that each generate a few binary artifact zip files
 (just click through via the build's title to see them).  The actions page is