README: Add link to spec.txt for finding out implemented spec version
README: Add link to spec.txt for finding out implemented spec version

atlassian/commonmark-java,vsch/flexmark-java,vsch/flexmark-java,commonmark/commonmark-java,vsch/commonmark-java,vsch/commonmark-java,vsch/flexmark-java,vsch/flexmark-java,jiakuan/commonmark-java,atlassian/commonmark-java,commonmark/commonmark-java,chiwanpark/commonmark-java,vsch/commonmark-java,atlassian/commonmark-java,chiwanpark/commonmark-java,chiwanpark/commonmark-java,jiakuan/commonmark-java,jiakuan/commonmark-java,commonmark/commonmark-java