Improved formatting of readme file

Added some better GitHub markdown formatting to the readme file in order to make it easier to read.

README.md

-minetest-caverealms
-===================
+# minetest-caverealms
 
 A mod for Minetest to add underground realms
 
+## Forum Topic
 For more information, view the official forum topic at:
-https://forum.minetest.net/viewtopic.php?f=9&t=9522
+- https://forum.minetest.net/viewtopic.php?f=9&t=9522
 
-Contributors:
-HeroOfTheWinds - everything
-|
-Zeno- - additional ideas and fine tuning
+## Contributors
+- HeroOfTheWinds - everything
+- Zeno - additional ideas and fine tuning
 
-Licensed under the WTFPL
+##Licensing
+- WTFPL
 
 Also be sure to check out VanessaE's texturepack "HDX", which supports this mod!