Add alt text to nodecg-io logo

Author: hlxid

docs/index.md

@@ -1,6 +1,6 @@
 # nodecg-io
 
-![](assets/header_white.png)
+![nodecg-io logo](assets/header_white.png)
 
 [![Feature Requests](https://img.shields.io/github/issues/codeoverflow-org/nodecg-io/enhancement?label=Feature%20Requests&style=flat-square)](https://github.com/codeoverflow-org/nodecg-io/labels/enhancement)
 [![Bugs](https://img.shields.io/github/issues/codeoverflow-org/nodecg-io/bug?label=Bugs&style=flat-square)](https://github.com/codeoverflow-org/nodecg-io/labels/bug)