add stackoverflow link

丰宝龙 fengbaolong (88342) · 丰宝龙 fengbaolong (88342) · commit d99820891671 · 2016-03-09T16:36:28.000+08:00
diff --git a/contents/how-can-i-convert-a-stack-trace-to-a-string.md b/contents/how-can-i-convert-a-stack-trace-to-a-string.md
@@ -33,4 +33,7 @@ public String stackTraceToString(Throwable e) {
     }
     return sb.toString();
 }
-````
+````
+
+stackoverflow链接：
+http://stackoverflow.com/questions/1149703/how-can-i-convert-a-stack-trace-to-a-string

Original file line number	Diff line number	Diff line change
`@@ -33,4 +33,7 @@ public String stackTraceToString(Throwable e) {`
`33`	`33`	`}`
`34`	`34`	`return sb.toString();`
`35`	`35`	`}`
`36`		-````
	`36`	+````
	`37`	`+`
	`38`	`+stackoverflow链接：`
	`39`	`+http://stackoverflow.com/questions/1149703/how-can-i-convert-a-stack-trace-to-a-string`