add notebook keywords reference

Author: jeffjennings

resources/keywords.md

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+# Astropy package keywords
+astroquery
+coordinates
+astroplan
+reproject
+table
+spectral cube
+aplpy
+imexam
+specutils
+photutils
+wcs
+units
+time
+synphot
+dust extinction
+modeling 
+convolution
+gala
+vo conesearch
+
+# Python package keywords
+matplotlib
+numpy
+scipy
+dask
+
+# Task keywords
+contour plots
+data cubes
+FITS
+image analysis
+image manipulation
+model fitting
+file input/output
+histogram
+error bars
+OOP
+scatter plots
+spectroscopy
+time series
+units
+Vizier
+photometry
+colorbar
+LaTeX
+modular code
+Simbad
+scalebar
+pv diagram
+
+# Science keywords
+astrodynamics
+stellar photometry
+stellar physics
+radio astronomy
+astrostatistics 
+x-ray astronomy
+galactic astronomy
+galaxy dynamics
+stellar evolution
+physics
+observational astronomy
+astrometry
+extinction
+polarimetry 
+IR and optical astronomy