Add example to overview section

README.md

@@ -64,7 +64,19 @@ md_tools.sh
 
 ## Overview
 
-Given a list of entityIDs and a metadata source, bash script ``check_idp_error_urls.sh`` probes each entity and determines which of the entityIDs correspond to SAML IdP deployments. For each such deployment, the script determines whether it has an ``errorURL`` in metadata and whether or not that URL is resolvable.
+Given a list of entityIDs and a metadata source, bash script ``check_idp_error_urls.sh`` probes each entity and determines which of the entityIDs correspond to SAML IdP deployments. For each such deployment, the script determines whether it has an ``errorURL`` in metadata and whether or not that URL is resolvable. For example:
+
+```Shell
+# Fetch the main production metadata aggregate at md.incommon.org:
+$ MD_LOCATION=http://md.incommon.org/InCommon/InCommon-metadata.xml
+$ MD_PATH=$MD_DIR/InCommon-metadata.xml
+$ /usr/bin/curl --silent $MD_LOCATION > $MD_PATH
+
+# Probe every errorURL in IdP metadata registered by InCommon
+$ /bin/cat $MD_PATH \
+  | /usr/bin/xsltproc $LIB_DIR/extract_InCommon_IdP_entityIDs.xsl - \
+  | $BIN_DIR/check_idp_error_urls.sh -f $MD_PATH
+```
 
 See the inline help file for details: