About generated masterkey, more how-to about subkey on keybase doc ?

Hi,

I have been using Keybase since few months using a simple use case (one master key to sign/encrypt), and now i want to go one step further in security of my system... In order to do that, i see on many security websites the need to create two or more subkeys based on master key.

If i read correctly the result of the `gpg -K` command on Ubuntu 17.10, the generated master-key/secret-key (sec) made using keybase has already one subkey (ssb)

```
/home/xxx/.gnupg/pubring.kbx
-------------------------------
sec rsa4096 2017-08-11 [SC] [expire : 2033-08-07]
82D906D9C4C2D3576E9EC380952B29FC7831465D
uid [ ultime ] xxx
ssb rsa4096 2017-08-11 [E] [expire : 2033-08-07]
```

I have two or more question based on my readings, and i doesn't found informations about that on keybase documentation :
- is it possible to change status of the sec key [SC] to [C] ? If yes, how can i do that ? And after this operation is it always possible for me to read the previously encrypted email using this secret-key (sec) ?
- is it possible to to create a new subkey [SC] to sign and encrypt my future mail based on this master-key ? I need to republish and share with my colleague the new private/public sub key generated after that ?

Thanks a lot for you insights on this "not so easy" subject.

Review the Keybase documentation related to the generated master key and the gpg -K output shown in the issue. Determine how changing the master-key capabilities and adding signing or encryption subkeys should be documented, including effects on existing encrypted mail and whether keys must be republished. Done means the documentation answers each of the listed questions with verified instructions.

Written by the indexing model from the issue text.