README: Add initial documentation

Document how to create a configuration file and what other dependencies
you might need to get everything working.

README.adoc

+Turris Webapps
+==============
+
+Simple landing page allowing users to choose which we application they want to
+continue to. List of available applications is read from directories
+`/usr/share/turris-webapps/` and `/etc/turris-webapps/`. Those can contain json
+files with definitions of the service. Files has to begin with number (to allow
+sorting them easily) and have `.json` extension.
+
+Web apps definition files
+-------------------------
+
+Json files with web app definitions look like this:
+
+[source,json]
+-------------------------------------------------------------------------------
+{
+    "id": "coolapp",
+    "title": "Cool App",
+    "url": "/coolapp",
+    "icon": "/icons/coolapp.svg",
+    "description": {
+	"en": "This is the coolest app ever",
+	"cz": "Úplně boží aplikace"
+    }
+}
+-------------------------------------------------------------------------------
+
+Regarding the records in the json, `id` is unique identifier, `title` should be
+name of the application - thus does not depend on any particular language. To
+specify where to redirect, use `url` entry. Preferred way is to specify
+different path on the same server/port and proxy the service if it runs on
+different port. Reason behind that is to make https available and to make sure
+everything works even if only one port is available. In worst case, you can use
+as part of `url` special string `@HOST` which will be replaced by the host name
+or IP of the current server, so you could something like `"url":
+"http://@HOST@:8080",`. To have a nice icon, you need to specify path to your
+icon by filling in the `icon` entry. Your icon should be stored on router in
+`/www/webapps-icons/` directory and would be accessible in webapps via
+`/icons`. Last but not least, `description` can be a short text to explain what
+the application does and it can be translated into various languages.
+
+Lighttpd configuration
+----------------------
+
+To create a reverse proxy in lighttpd to support web application running on
+different port (optionally with password protected access), you need to create
+and deploy configuration file into `/etc/lighttpd/conf.d/`. Name it
+_your_app_name.conf_. As a content you use the following snippet:
+
+-------------------------------------------------------------------------------
+server.modules += ( "mod_proxy" )
+
+$HTTP["url"] =~ "^/coolapp" {
+  proxy.header = (
+    "map-host-request" => ( "-" => "127.0.0.1" ),
+    "map-urlpath" => ( "/coolapp" => "", "/coolapp/" => "/" ),
+    "https-remap" => "enable"
+  )
+  proxy.server = ( "" => ( ( "host" => "127.0.0.1", "port" => "1234") ) )
+}
+-------------------------------------------------------------------------------
+
+If you want to protect the patch by password, you can expand it this way:
+
+-------------------------------------------------------------------------------
+server.modules += ( "mod_proxy" )
+server.modules += ( "mod_authn_pam", "mod_auth" )
+
+$HTTP["url"] =~ "^/coolapp" {
+  auth.backend = "pam" 
+  auth.require = ( "" =>
+                   (
+                     "method"    => "basic",
+                     "realm"     => "Cool App login",
+                     "require"   => "valid-user" 
+                   )
+                 )
+  proxy.header = (
+    "map-host-request" => ( "-" => "127.0.0.1" ),
+    "map-urlpath" => ( "/coolapp" => "", "/coolapp/" => "/" ),
+    "https-remap" => "enable"
+  )
+  proxy.server = ( "" => ( ( "host" => "127.0.0.1", "port" => "1234") ) )
+}
+-------------------------------------------------------------------------------
+
+Dependencies
+~~~~~~~~~~~~
+
+Obviously configuration above depends on `lighttpd` and various modules. For
+simple proxy without authentication you are fine with just
+`lighttpd-mod-proxy`. If you need authentification, you have to add to your
+dependencies `lighttpd-mod-auth`, `lighttpd-mod-authn_pam` and
+`lighttpd-mod-authn_file` as well.