Add global from to Mailing docs

[lwestfall]

Describe the solution you'd like
The documentation (great as it is) is missing details about the usage of globalFrom in service registration which would've sped up development for my use case. After digging into the source code I have a good handle on it. If you'd like I'd be happy to take a stab at a PR!

[jamesmh]

Cool! Would love the help. You can have a look at this file (the docs for the mailer):

https://github.com/jamesmh/coravel/blob/master/DocsV2/docs/Mailing/README.md

[lwestfall]

Sounds good!

One quick question, I noticed that calling .From("someone@somewhere.com") in Build() doesn't seem to override the globalFrom address. Would it be better if it did? In my mind it seems like globalFrom should sort of be treated as a "default" from address, which can be overridden when building (maybe someone wants to conditionally override the from address, or certain Mailable implementations should have a different From address, etc).

I know this is a little more involved than just the docs so if it's something you're interested in I'll get this into a separate issue and PR.

[jamesmh]

Ya I think that makes sense to override the global from if you call From() 👍