Update index.html to clarify template precompilation

The example code for template precompilation using _.template().source is unclear to those unfamiliar with precompilation:

```
<script>
  JST.project = <%= _.template(jstText).source %>;
</script>
```

This fix changes adds two things to clarify. It adds a type attribute to the script tag to clarify its use and make similar examples easier to find, and it adds a brief description of the example so it is more clear what is being shown (changes below):

```
For example, using a pre-compiler with ERB-style delimiters:

<script>
  JST.project = <%= _.template(jstText).source %>;
</script>
```