Clarify developer installation instructions, and fix up some metadata.

[ceball]

• Clarify developer installation instructions.
• Fix nodejs dependency (I don't think it is required to run the tests...right? It is required for developers, but they are instructed to install the build extra, which includes nodejs.)

The changes to the developer guide should definitely be reviewed. Also, I've never created or edited a panel model (as far as I can recall!), so I'm sure someone could add something more useful...

Also, because I was in there:

• Update license years and team name.
• Update package metadata.
• Removed postBuild (am I right it's unused?)

[philippjfr]

The postBuild should have a note in it to mention that it's used for binder. It is indeed still needed for that reason.

[codecov]

Codecov Report

Merging #978 into master will increase coverage by 0.32%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #978      +/-   ##
==========================================
+ Coverage   85.74%   86.06%   +0.32%     
==========================================
  Files          98       99       +1     
  Lines       11072    11665     +593     
==========================================
+ Hits         9494    10040     +546     
- Misses       1578     1625      +47

Impacted Files	Coverage Δ
setup.py	0% <ø> (ø)	⬆️
panel/io/state.py	94.23% <0%> (-0.9%)	⬇️
panel/template.py	69.59% <0%> (-0.78%)	⬇️
panel/tests/conftest.py	98.01% <0%> (-0.2%)	⬇️
panel/tests/test_config.py	100% <0%> (ø)
panel/pane/markup.py	97.94% <0%> (+1.98%)	⬆️
panel/util.py	94.52% <0%> (+2.73%)	⬆️
panel/config.py	55.02% <0%> (+3.05%)	⬆️
panel/viewable.py	85.17% <0%> (+3.65%)	⬆️
panel/io/save.py	85.24% <0%> (+7.24%)	⬆️
... and 1 more

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[philippjfr]

Thanks, looks good to me!