Added readme notes how to add source repository for DEB822-source format

[easymoney322]

Hello,
I've made some changes to README file.

Current stable LTS distributives of Ubuntu 24.04 and Debian 12 are using the new file format for apt source lists.

The changes are both the new system-repo source list file location, and the format.

In case of ubuntu, the default file is located in /etc/apt/sources.list.d/ubuntu.sources and the default content is this:

Types: deb
URIs: http://archive.ubuntu.com/ubuntu
Suites: noble noble-updates noble-backports
Components: main restricted universe multiverse
Signed-By: /usr/share/keyrings/ubuntu-archive-keyring.gpg

Types: deb
URIs: http://security.ubuntu.com/ubuntu
Suites: noble-security
Components: main restricted universe multiverse
Signed-By: /usr/share/keyrings/ubuntu-archive-keyring.gpg

So to add sourcetree it should be changed to this:

Types: deb deb-src
URIs: http://archive.ubuntu.com/ubuntu
Suites: noble noble-updates noble-backports
Components: main restricted universe multiverse
Signed-By: /usr/share/keyrings/ubuntu-archive-keyring.gpg

Types: deb deb-src
URIs: http://security.ubuntu.com/ubuntu
Suites: noble-security
Components: main restricted universe multiverse
Signed-By: /usr/share/keyrings/ubuntu-archive-keyring.gpg

This might not be obvious for some people given the new format.

I'm not sure if I should rewrite step 2 for both ubuntu and debian, given that some people are still using older systems.

So I've made a note about this in troubleshooting section, that is related for missing kernel sourcetree.

[ja49619]

WHY you block this commit? These comments would have saved me a lot of hours of searching for the problem of your incorrect instructions

[tiaga]

However, old format works well at least on Ubuntu 24.04.

[stone-w4tch3r]

+1 for this PR. Documentation definetly will benefit from this, especially for newbie users (as in #29)

[AdamsGH]

@easymoney322 I've already lost a few days with this issue on AWS. Many thanks you saved another one