change Migration Guide to markdown (#305)

Author: jmckenna

MIGRATION_GUIDE.txt MIGRATION_GUIDE.mdMIGRATION_GUIDE.txt renamed to MIGRATION_GUIDE.md

@@ -26,7 +26,7 @@ Migrating from MapCache 1.6 to 1.8
 * <dimensions type="time" ...>...<query>SQL</query> should be replaced by
   <dimensions type="time" ...>...<validate_query>SQL</validate_query><list_query>...</list_query> or
   <dimensions type="sqlite" time="true" ...>...<validate_query>SQL</validate_query><list_query>...</list_query>
-  (see [RFC-121](http://mapserver.org/development/rfc/ms-rfc-121.html) for full examples)
+  (see [RFC-121](https://mapserver.org/development/rfc/ms-rfc-121.html) for full examples)
 
 Migrating from MapCache 1.4 to 1.6
 ==================================

release.sh

@@ -26,7 +26,7 @@ fi
 echo "#"
 echo "# Make sure:"
 echo "# - you are on branch-$ms_version_major-$ms_version_minor"
-echo "# - you have edited MIGRATION_GUIDE.txt with changes related to this release"
+echo "# - you have edited MIGRATION_GUIDE.md with changes related to this release"
 echo "#"
 echo ""