Add instructions for submodule
Add instructions for submodule

Otherwise it fails with

```
  Conversion error: Jekyll::Converters::Scss encountered an error while converting 'css/main.scss':
                    File to import not found or unreadable: bootstrap/scss/bootstrap.scss. Load path: /Users/vjeux/random/website/_sass on line 9
jekyll 3.2.1 | Error:  File to import not found or unreadable: bootstrap/scss/bootstrap.scss.
Load path: /Users/vjeux/random/website/_sass on line 9
```
elwayman02/website-1,elwayman02/website-1,elwayman02/website-1