Documentation improvement: styles.css requirement

[robbienohra]

It should be made more clear in the documentation that the following import is required:

import 'react-base-table/styles.css'

I.e. that BaseTable will not render correctly without the above import.

It's not clear that BaseTable will not render correctly "out of the box" without the above style import.

[nihgwu]

PR is welcome :p

[robbienohra]

@nihgwu least I can do for the fabulous work you all have done with this library! I'll get on it 😉

[robbienohra]

Done! Let me know if I should change the phrasing at all. 😊

[robbienohra]

It's not really the styles.css file per se as much as it is the CSS classes defined in them, lol. So maybe it's better to phrase it to emphasize the classes themselves. Open to feedback!