Add while-read example to read docs

doc_src/read.txt

@@ -51,4 +51,9 @@ The following code stores the value 'hello' in the shell variable `$foo`.
 
 \fish
 echo hello|read foo
+
+# This is a neat way to handle command output by-line:
+printf '%s\n' line1 line2 line3 line4 | while read -l foo
+                  echo "This is another line: $foo"
+              end
 \endfish