Conver API to markdown format for better readability.

README.txt.md

@@ -67,7 +67,7 @@ ready, run the plot function:
 Here, data is an array of data series and options is an object with
 settings if you want to customize the plot. Take a look at the
 examples for some ideas of what to put in or look at the reference
-in the file "API.txt". Here's a quick example that'll draw a line from
+in the file "API.txt.md". Here's a quick example that'll draw a line from
 (0, 0) to (1, 1):
 
     $.plot($("#placeholder"), [ [[0, 0], [1, 1]] ], { yaxis: { max: 1 } });