Hello World for DigitalOut

Revision:
11:759df8b71f32
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/.hgignore	Thu Aug 13 16:16:51 2020 +0800
@@ -0,0 +1,3 @@
+^BUILD$
+^.mbed$
+^mbed-os$