Fix headings in README

In its current form, the `README.mkdn` is a little odd because the headings for the individual distro instructions are larger than the heading that introduces the installation section.

Basically, this in Markdown:

```
Arch Linux
============
```

is larger than this:

```
How do I get screenFetch?
---------------------
```

So I've fixed this by making the individual distro headings `h3`, which can't be accomplished by using hyphens/equal signs. As such, `#`s had to be used.

You can see what this new `README.mkdn` looks like [here](https://github.com/KenanY/screenFetch/tree/b11d665e00f5ba4b24188d7e9bbc8fdca6cbf6dd).