Doc: Add upgrade instructions

[adrienandrem]

What's the problem you want solved?

Benefit from bugfixes with no code knowledge.

Is there a solution you'd like to recommend?

README.md mentions "upgrade often".
In install.md, develop this with explicit shell command(s).

[trosel]

How does one upgrade the app in general (Without doing an uninstall and re-install that loses your data)?

Or, for faster updates and less stability, install Oasis from GitHub and upgrade often.

If one would do this advanced method, this is how I understand it:

Install first

git clone https://github.com/fraction/oasis.git
cd oasis
npm install -g --only=prod

Then to run oasis, just type oasis (do we need to add this to the path variables?)

Then to upgrade, cd into the oasis top level directory and run

git pull
npm install -g --only=prod

Is that right?