Update README: installation instructions and development team

[ali-ramadhan]

No description provided.

[glwagner]

How about changing

Installing Oceananigans through the built-in Julia package manager is a good idea because Julia will take care of all dependencies and let you update Oceananigans using a single command: ] update.

to something like

"We recommend installing Oceananigans.jl with the built-in Julia package manager, because this installs a stable, tagged release.

Oceananigans.jl can be updated to the latest tagged release from the package manager by typing

(v1.1) pkg> update Oceananigans

At this time, updating Oceananigans.jl should be done with care, as Oceananigans.jl is under rapid development and breaking changes to the user API occur often."

Do we want to also move the "Developers" section to the end of the README? Maybe expand the scope of this PR slightly while we are looking at it?

Edit: maybe we should also provide links to the julia documentation on how to use the built-in package manager?

[codecov]

Codecov Report

Merging #396 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #396   +/-   ##
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  Coverage   64.62%   64.62%           
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  Files          23       23           
  Lines        1405     1405           
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  Hits          908      908           
  Misses        497      497

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[codecov]

Codecov Report

Merging #396 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #396   +/-   ##
=======================================
  Coverage   64.62%   64.62%           
=======================================
  Files          23       23           
  Lines        1405     1405           
=======================================
  Hits          908      908           
  Misses        497      497

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 079cb53...dd2f7f6. Read the comment docs.