Add cc-by-sa license for advisories and patches (#263)

* Add CC-by-sa license

* Add licence for code advisories under _posts sub directory

LICENSE.txt

@@ -1,3 +1,7 @@
+===
+This license applies to the entire repo except for subfolders that have their own license file. In such cases, the license file in the subfolder takes precedence.
+===
+
 The MIT License (MIT)
 
 Copyright (c) 2016-present Parker Moore and the minima contributors

_layouts/post.html

@@ -47,3 +47,4 @@ <h1 class="post-title p-name" itemprop="name headline">{{ page.title | escape }}
 <p style="margin-top: 20px; font-size:80%">
   <strong>DISCLAIMER:</strong> <em>The French Association Friends Of Presta (FOP) acts as an intermediary to help hosting this advisory. While we strive to ensure the information and advice provided are accurate, FOP cannot be held liable for any consequences arising from reported vulnerabilities or any subsequent actions taken.</em>
 </p>
+<p xmlns:cc="http://creativecommons.org/ns#" >This advisory and patch is licensed under <a href="https://creativecommons.org/licenses/by-sa/4.0/?ref=chooser-v1" target="_blank" rel="license noopener noreferrer" style="display:inline-block;">CC BY-SA 4.0<img style="height:22px!important;margin-left:3px;vertical-align:text-bottom;" src="https://mirrors.creativecommons.org/presskit/icons/cc.svg?ref=chooser-v1"><img style="height:22px!important;margin-left:3px;vertical-align:text-bottom;" src="https://mirrors.creativecommons.org/presskit/icons/by.svg?ref=chooser-v1"><img style="height:22px!important;margin-left:3px;vertical-align:text-bottom;" src="https://mirrors.creativecommons.org/presskit/icons/sa.svg?ref=chooser-v1"></a></p>