README: add requirements and choose Turris OS version

Add TIP and remove the section with make menuconfig as it is no longer
necessary

README.asciidoc

@@ -6,19 +6,41 @@ Getting started
 
 Create an empty directory you want to build Turris OS in, enter that directory and
 from it call `compile_pkgs` script. In most basic use-case, this should be enough.
-Be aware - *it will delete all previous content of the current directory*. You can
-repeat individual steps of the compilation separately, check with the following
+
+[TIP]
+*It will delete all previous content of the current directory*.
+
+You can repeat individual steps of the compilation separately, check with the following
 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.
+
+Requirements
+-------------
+
+You need to have a GNU/Linux distribution to be able to compile Turris OS and packages.
+
+To install all dependencies for Debian distribution you can use:
+
+--
+  apt install ca-certificates git build-essential zlib1g-dev gawk libssl-dev subversion unzip libncurses-dev wget python file rsync
+--
+
+For other distributions, we suggest to look at https://openwrt.org/docs/guide-developer/build-system/install-buildsystem[OpenWrt documentation].
 
 Advanced usage
 --------------
 
-Building a single package
-~~~~~~~~~~~~~~~~~~~~~~~~
+Building packages
+~~~~~~~~~~~~~~~~~~
 
-To build only one package you have to first prepare build directory. That can be
+To build a package you have to first prepare build directory. That can be
 achieved by running in target directory.
 --
  compile_pkgs prepare_tools -t board # <1>
@@ -28,9 +50,7 @@ achieved by running in target directory.
 
 Be aware *it removes previous content of current directory*!
 
-Once, you have compiled required tools, you need **enter build directory** and
-then run `make menuconfig`, select the package you want to compile and save it.
-Then you can build it using this command: +
+Once, you have compiled required tools, you can build packages using this command:
 --
  make package/name/compile # <2>
 --