Extend readme to contain layout of this repository

This should help newcomers with orientation in repository.

This also adds short readme files all over repository to document
content of various directories.

README.adoc

-Simple Turris OS building script
-================================
+Turris OS building scripts
+==========================
+
+Turris OS is based on OpenWrt. This repository contains script and additional
+patches to convert OpenWrt tree to Turris OS one.
+
+OpenWrt consists of core repository and additional packages feeds defined in `feeds.conf`.
+
+Repository layout
+-----------------
+
+`compile_pkgs`:: script to prepare OpenWrt based source tree and to compile
+  Turris OS in it. The final result are Turris OS packages.
+`generate_lists`:: this script generates lists for Turris updater (updater-ng).
+  Those are exposed alongside of Turris OS packages and control updater's
+  behavior.
+`generate_medkit`:: script creating Turris medkit (factory image) from Turris OS
+  packages and Turris updater's lists.
+`configs/`:: this directory contains configurations inserted to generated OpenWrt
+  configuration. There are subdirectories for every Turris router and directory
+  `common` that applies on all of them.
+`feeds.conf`:: this file defines which feeds and in what version are about to be
+  used for Turris OS packages compilation (OpenWrt based build). There is also
+  non-standard commented line for OpenWrt used by `compile_pkgs` script to get
+  OpenWrt source tree.
+`patches`:: this directory contains all patches applied on both base OpenWrt tree
+  as well as on any feed. It contains additional directories named after feed.
+  Given patch is applied on a feed of the same name as is name of directory it is in
+  relative to `patches/`. There can optionally be additional directories named after
+  the feeds to sort patches to groups. Please read
+  link:patches/README.adoc[readme file] in `patches` directory for more in depth
+  explanation.
+`lists/`:: directory with sources used in script `generate_lists` to generate
+  updater's lists.
+`NEWS`:: this is Turris OS changelog. It is parsed and used in git tags or in
+  package informing users about changes.
+`helpers/`:: directory containing libraries and utility scripts for top level
+  scripts and some less important scripts as well.
 
 Getting started
 ---------------
@@ -82,7 +118,7 @@ Development and maintenance
 Configuration
 ~~~~~~~~~~~~~
 
-Default config for OpenWRT is put in `configs` directory either in directory
+Default config for OpenWrt is put in `configs` directory either in directory
 `common` or in a directory named after the target board. Files in those
 directories get merged (common ones first) and will form the final `.config`
 file. Some parts of `compile_pkgs` might modify it further.

patches/README.adoc

+Patches
+=======