Add debug hints to show up in online editor (#614)

Author: mk-mxp

exercises/shared/.docs/debug.md

@@ -0,0 +1,9 @@
+# Debug
+
+When a test fails, a message is displayed describing what went wrong and for which input.
+
+Printing values and contents of variables can also help with understanding what happens.
+PHP can print values using:
+
+- `echo 'Debug message';` to output a string of your choice
+- `var_dump($variable);` to get more insight into a value, e.g. the type PHP sees