Upgrade the migration page, include VCL version table

This should allow us to close issue 3065 in Varnish Cache.

R1/source/tips/migration/index.rst

@@ -3,10 +3,56 @@
 Migrating Varnish versions
 ==========================
 
-XXX: This page is just a skeleton so far, please add tips and details that
-will be useful to users who will be migrating.
+This page is a continuous work in progress, and contributions to this
+is very much welcomed. Do not hesitate to open a "pull request" on
+`our GitHub repository <https://github.com/varnishcache/homepage/>`.
+
+The following resources documents upgrades from unsupported versions:
 
 * `Migrating from 3.0.x to 4.0.x
   </docs/4.1/whats-new/upgrade-4.0.html>`_
-* `Migrating to 4.1.x
-  </docs/4.1/whats-new/upgrading.html>`_
+* `Migrating to 4.1.x </docs/4.1/whats-new/upgrading.html>`_
+* `Upgrading to 5.0 </docs/6.0/whats-new/upgrading-5.0.html>`_
+* `Upgrading to 5.1 </docs/6.0/whats-new/upgrading-5.1.html>`_
+* `Upgrading to 5.2 </docs/6.0/whats-new/upgrading-5.2.html>`_
+* `Upgrading to 6.0 </docs/6.0/whats-new/upgrading-6.0.html>`_
+
+Releases after 6.0 do not come with Long Time Support, but are
+released as "fresh". A "fresh" release will get security updates for a
+while, but not general bug fixes. A release can be promoted to LTS in
+a separate announcement, and it will then receive general bug fixes
+and some new features from newer releases and development.
+
+Updating to the latest releases are documented here:
+
+* `Upgrading to 6.1 </docs/trunk/whats-new/upgrading-6.1.html>`_
+* `Upgrading to 6.2 </docs/trunk/whats-new/upgrading-6.2.html>`_
+* `Upgrading to 6.3 </docs/trunk/whats-new/upgrading-6.3.html>`_
+
+Upgrading from 4.x to the latest LTS should be very straightforward,
+and require much less effort than going to 3.x to 4.x.
+
+Versions of the VCL language
+============================
+
+Currently there are two official versions of the VCL language, 4.0 and
+4.1, and they are only slightly different.
+
+This table shows which versions of the VCL language are supported in
+different versions of Varnish:
+
+=============== ======================
+Varnish version Supported VCL versions
+=============== ======================
+4.0*            4.0
+4.1*            4.0
+5.0*            4.0
+5.1*            4.0
+5.2*            4.0
+6.0 LTS         4.0, 4.1
+6.1*            4.0, 4.1
+6.2*            4.0, 4.1
+6.3             4.0, 4.1
+=============== ======================
+
+The versions marked with a star (*) are End Of Life.