debian: README.source: update and fix some packaging information

debian/README.source

@@ -14,7 +14,7 @@ Downloading the sources
 -----------------------
 Our kernel tree is hosted at https://source.puri.sm/Librem5/linux-next
 where you can clone it from. The default branch is called
-'pureos/amber-phone'.
+'pureos/byzantium'.
 
 How to build the kernel
 -----------------------
@@ -68,7 +68,7 @@ to actually submit patches upstream too.
 
 Release process
 ---------------
-Changes that are merged into the default branch 'pureos/amber-phone'
+Changes that are merged into the default branch 'pureos/byzantium'
 in general can be released (=tagged) at any point in time. In this case a
 new version of the 'linux-librem5' package gets created. In case no ABI change
 is to be expected (the usual case), only the versions' last digit
@@ -91,15 +91,14 @@ The kernel image's Debian package name contains a version in order to be
 able to create an entirely new package when the ABI to userspace
 changes (should be avoided though) or we update to a new major version
 of the kernel, see below. That number (name) is defined in debian/rules, for
-example 'KREL=5.11.0-1-librem5'. After changing this, regenerate
+example 'KREL=5.15.0-1-librem5'. After changing this, regenerate
 debian/control too by running 'debian/rules debian/control'.
 
 The actual version of that Debian package is defined in debian/changelog.
 First, use the upstream version that the tree is based on, for example
-'5.11.0~rc4pureos0' for a tree based on v5.11-rc4 or
-'5.11.3pureos0' for a tree based on v5.11.3. When doing a release
+'5.13.10pureos1' for a tree based on v5.13.10. When doing a release
 that doesn't include an update from upstream, then just increment the last
-digit: '5.11.3pureos1'.
+digit: '5.13.10pureos2'.
 
 Update process
 --------------
@@ -115,12 +114,12 @@ things are currently a bit more complicated:
 In an ideal world, an update from one Linux major version to the next
 would similarly be a merge and some semi-automated testing of all needed
 functionality. The world is not ideal yet though, so when a new
-major kernel version is tagged upstream (usually an "rc" tag already)
+major kernel version is tagged upstream (usually an "rc" tag)
 we rebase our changes onto it, test as much as we reasonably can
 (see below) and tag a release of the 'linux-next-librem5' Debian package
 for wider testing. That new tree is being prepared (and rebased) on the
-'next/amber-phone' branch. It should have the same functionality
-as the default 'pureos/amber-phone' branch, only rebased
+'next/byzantium' branch. It should have the same functionality
+as the default 'pureos/byzantium' branch, only rebased
 (and cleaned up) onto a newer mainline release.
 
 Testing
@@ -146,8 +145,8 @@ PureOS distributions
 This kernel is released to the amber-phone PureOS distribution (based on
 Debian buster) and to the byzantium PureOS distribution (based on bullseye).
 As suggested by the default branch, changes are usually merged into
-pureos/amber-phone first. There is also the pureos/byzantium branch that
-can contain changes that depend on having the newer userspace environment.
+pureos/byzantium first. The pureos/amber-phone branch contains changes on
+top, in order to support the older userspace environment.
 Other than that, the kernels are the same but the respective branches
 are of course used to release to the distributions.
-For amber-phone, append "~amberN" to the version number.
+For amber-phone, append "~amber1" to the version number.