Add CLI instructions for accepting invite to the Invitation Email

I was invited to join keybase.io and I am excited.
This is very much what I need for a project I am involved with.

The invite email states that I can accept the invitation by clicking the link, or by using the keybase app.

Naturally, I decided to test the second option, assuming that it is the road less taken.
I installed the windows app and launched it, but typing the invitation code at it's command line tells me that:

```
'[The first word of the invite code]' is not recognized as an internal or external command,
operable program or batch file.
```

I tried typing help on the command line, but none of the commands listed looks like it does what I'm looking for.

The answer of course is to do

```
keybase signup
```

and be led through the process. However, it would be nice to be told that in the email.

I look forward to seeing the new features as they are added and seeing where this project goes.

Start with the invitation email text and the `keybase signup` entry point described in the issue. Update the email to explain that invitees can accept through the app using that command, then verify the instructions are clear.

Written by the indexing model from the issue text.