Add note regarding Artisan's CLI directory.

I had a blonde moment and forgot that, to access a shell script, one needs to be in its directory, or provide the full path. I'm sure others not as familiar as me will have the same hiccup at some point, so a note is nice to have.

I'm not 100% sure if the wording is best for "beginners" or what have you, but I figure a lot of Laravel isn't geared towards them in the first place, so I didn't rack my brain over it. Suggestions are welcome.