SLING-7167 Adjust READMEs

switch from plain text to Markdown

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1810546 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/README.txt b/README.md
similarity index 100%
rename from README.txt
rename to README.md