docs: link logo at the top with the official website (#84)

When I was reading README.md,
I clicked the logo and I expected to go to
the official website https://flarum.org/
but it didn't. So, the image is being linked
to the official website in this commit.

Signed-off-by: Soobin Rho <soobinrho@gmail.com>

README.md

@@ -1,4 +1,6 @@
-<p align="center"><img src="https://flarum.org/assets/img/logo.png"></p>
+<p align="center">
+<a href="https://flarum.org/"><img src="https://flarum.org/assets/img/logo.png"></a>
+</p>
 
 <p align="center">
 <a href="https://packagist.org/packages/flarum/core"><img src="https://poser.pugx.org/flarum/core/d/total.svg" alt="Total Downloads"></a>