fix for https://github.com/Microsoft/pxt/issues/549

docs/reference/serial/read-until.md

@@ -0,0 +1,28 @@
+# Serial Read Until
+
+Read a text from the serial port until a delimiter is found.
+
+```sig
+serial.readUntil(",");
+```
+
+### Returns
+
+* a [string](/reference/types/string) containing input from the serial port, such as a response typed by a user
+
+### Example
+
+The following example reads strings separated by commands (``,``).
+
+```blocks
+basic.forever(() => {
+    let answer = serial.readUntil(",");
+    serial.writeLine(answer);
+});
+```
+
+### See also
+
+[serial](/device/serial),
+[serial write line](/reference/serial/write-line),
+[serial write value](/reference/serial/write-value)