README: update section about releases to be more general

The original text was little bit cryptic and wasn't true in the end
because master is now considered v6.0 and this of course changes as we
go trough new versions. This should now explain reader how to get
specific version in more general terms.

README.asciidoc

@@ -14,12 +14,19 @@ You can repeat individual steps of the compilation separately, check with the fo
 command `compile_pkgs --help` for a list of available commands, generic options,
 and their brief descriptions.
 
-Before you started you need to choose if you want to build
-
-* stable release of OpenWrt (Turris OS 4.x), you need to checkout for `v4.0` branch
-* master release of OpenWrt (Turris OS 5.x), you need to checkout for `master` branch.
-
-To use specific stable release you have to checkout that specific tag. Please see all available tags in repository.
+Before you started you need to choose what version of Turris OS you want to
+build. There are development versions in branches named after future release.
+Those are for example `v4.0` for Turris OS 4.0.X releases or `v4.1` for Turris OS
+4.1.X releases. Then there are old releases that are tags. Those are in format of
+for example `v4.0.0` for Turris OS 4.0.0 or `v4.1.2` for Turris OS 4.1.2.
+
+To get to appropriate version you have to run command like this:
+`git checkout v4.0` where in this case this takes you to development branch for
+Turris OS 4.0.X versions.
+
+It is advised to build the latest release version as with that you are most likely
+to get to the end. To get latest version you can do:
+`git checkout $(git tag | sort | tail -1)`.
 
 Requirements
 -------------