SLING-7167 Adjust READMEs

switch from plain text to Markdown

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1810525 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/custom-selector-login-form/README.txt b/custom-selector-login-form/README.md
similarity index 100%
rename from custom-selector-login-form/README.txt
rename to custom-selector-login-form/README.md