Update license conditions in README.md.

README.md

@@ -1,3 +1,8 @@
-This is a repository of Citation Style Language (CSL) 1.0 styles.
-For more information about the repository, see https://github.com/citation-style-language/styles/wiki.
-For general information about the CSL project, see http://citationstyles.org.
+CSL 1.0 Style Repository
+========================
+
+https://github.com/citation-style-language/styles is the main repository for CSL 1.0 styles and is maintained by the Citation Style Language (CSL) project members.
+
+All the styles in this repository are released under the Creative Commons Attribution-ShareAlike 3.0 Unported license (http://creativecommons.org/licenses/by-sa/3.0/). For attribution, any software using CSL styles from this repository must include a clear mention of the CSL project and a link to the project's website. When distributing these styles, keep the listings of authors and contributors in the style metadata intact. 
+
+For more information, check out the project website at http://citationstyles.org/ and the repository wiki at https://github.com/citation-style-language/styles/wiki.