Clarify dnx go default repo/branch/file behavior in docs

Updated readme.md to specify that dnx go defaults to github.com, the main branch, and program.cs or the first .cs file as the entry point when running code from a public repository. This clarifies the default host and entry file selection for users.

Author: kzu

readme.md

@@ -68,7 +68,7 @@ Instead of a local `.cs` file, you can pass a remote reference. The tool will do
 the content (when needed) and treat the resulting local file as the entry point:
 
 ```console
-# Run from a public repo (defaults to main + program.cs or first .cs)
+# Run from a public repo (defaults to github.com, main + program.cs or first .cs)
 dnx go kzu/sandbox
 
 # Specific branch/tag and file