SLING-7167 Adjust READMEs

switch from plain text to Markdown

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1810529 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/post-servlet-extensions/README.txt b/post-servlet-extensions/README.md
similarity index 100%
rename from post-servlet-extensions/README.txt
rename to post-servlet-extensions/README.md