Added a README.md

Added a README.md and an image for the README.
This README is a simple way of informing the user on how to use the extension when they are editing a wiki-page.

Author: QuickWrite

README.md

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+# MultiCodeBlock
+Creates a codeblock consisting of multiple languages and a special way of commenting the code.
+
+![The MultiCodeBlock itself](./img/MultiCodeBlockPreview.png "A preview of the MultiCodeBlock")
+
+## How it works
+
+The input is structured in a HTML-Style:
+* `<multicodeblock>` - This is the main body of the codeblock and directly calls `MultiCodeBlock::renderMultiCodeBlock` by MediaWiki
+    * `<codeblock lang="<language>">"` - These are the different codeblocks (languages). The order of these codeblocks decides the order in the navigation. The `language` is the filetype and not the name of the programming language itself, but it is possible to use the name.
+        * `<code>` - This is the code itself and is going to be highlighted.
+        * `<desc>` - The description is a block of different blocks of the description.
+            * `<position line="<line-number>">` - The positions are holding the description itself the the starting point of the description (relative to the code). It is possible to write in Wiki-Syntax in these.
+
+So it would be something like this:
+```html
+<multicodeblock>
+    <codeblock lang="cpp">
+        <code>
+#include <iostream>
+ 
+int main() {
+  std::cout << "Hello, World!" << std::endl;
+
+  return 0;
+}
+        </code>
+        <desc>
+            <position line="3">
+                '''This''' is the <code>main</code>-function
+            </position>
+            <position line="5">
+                Here stops everything.
+            </position>
+        </desc>
+</multicodeblock>
+```
+
+Currently the tabs in the code are rendered in the browser and so there shouldn't be and unnecessary tabs. This creates the break in the `<code>`-block.
+
+----
+
+This codeblock is an element where the different versions of the code are accessible through a navigation on the left side.
+The codeblocks itself are a table that has the code on the left side and the comments on the right side. The Code is seperated into rows with a simple ordered list that starts on each table cell with a specific value.
+
+To avoid problems with selecting the code there are two ways of copying it: The first way is the raw selection that is made possible with JavaScript and the constant removal and addition of the `user-select: none;` css property and the second way is by clicking the button on the right side that is copying the code and is directly adding it into the clipboard.
+
+## License
+The project is licensed under the permissive Apache 2.0 License.