Added sample-.hgignore file to show users how to create an .hgignore

specification for generated files or directories not of interest to them.

sample-.hgignore

+# You can copy and/or modify this file to create your own .hgignore file.
+# This keeps generated files from being added to source control management.
+^\.hgignore
+^appinventor/build/
+^appinventor/reports/
+^appinventor/appengine/build
+^appinventor/appengine/reports
+^appinventor/buildserver/build
+^appinventor/buildserver/reports
+^appinventor/blockseditor/build
+^appinventor/blockseditor/reports
+^appinventor/blockslib/build
+^appinventor/blockslib/reports
+^appinventor/common/build
+^appinventor/common/reports
+^appinventor/components/build
+^appinventor/components/reports
+