Add screenshot of Medik skin to README

Screenshot is useful for people who want to see how the skin looks like
before downloading and installing that - it's only additional ~70 kB, so
OK.

Additionaly fix some typos in README and move stuff to 'assets/' folder.

Signed-off-by: Petr Kajzar <petr.kajzar@lf1.cuni.cz>

COPYING

@@ -4,7 +4,7 @@ license:
 	SPDX-License-Identifier: CC0-1.0
 
 For the full license text please see 'license-cc0.txt' file delivered with this
-skin.
+skin in the 'assets/' folder.
 
 Note that the skin is:
 

README.md

@@ -5,13 +5,15 @@ _MediaWiki skin for WikiSkripta.eu._
 The Medik skin is a simple Bootstrap-based skin originally created for
 [WikiSkripta](https://www.wikiskripta.eu).
 
+![Medik](assets/medik.png)
+
 ## Installation
 
 * Download and place the file(s) in a directory called Medik in your `skins/`
   folder.
 * Add the following code at the bottom of your `LocalSettings.php`:
 
-		wfLoadSkin( 'Medik' );
+    wfLoadSkin( 'Medik' );
 
 * Done - Navigate to `Special:Version` on your wiki to verify that the skin is
   successfully installed.
@@ -109,4 +111,4 @@ To the extent possible under law, I have dedicated all copyright and related
 and neighboring rights to this software to the public domain worldwide. This
 software is distributed without any warranty.
 
-For additional info please refer to the [COPYING](COPYING file).
+For additional info please refer to the [COPYING file](COPYING).