Update README.md file to include notebook categories

[mackjenn]

Let's try to update this once Annie's notebooks are finished and before the STAN goes out in ~2 weeks. Suggested organization:

General Tools: • WFC3 Image Displayer and Analyzer • Exception Report Checklist - WFC3 • Manual Recalibration with calwf3 • Processing WFC3/UVIS Data with calwf3 Using the v1.0 CTE-Correction

IR Notebooks: • IMA Viewer • Masking Persistence in WFC3/IR Images • How to use wfc3_dash on DASH data

IR Variable Background : • IR IMA Visualization Tools with An Example of Time Variable Background • Manual Recalibration with calwf3: Turning off the IR Linear Ramp Fit • Correcting for Helium Line Emission Background in WFC3/IR Exposures using the "Flatten-Ramp" Technique • Correcting for Scattered Light in IR Exposures: Manually Subtracting Bad Reads • Correcting for Scattered Light in IR Exposures: Using calwf3 to Mask Bad Reads

Photometry : • WFC3/UVIS Filter Transformations with stsynphot • Flux Unit Conversions with synphot and stsynphot • Synthetic Photometry Examples for WFC3 WFC3/UVIS Time-dependent Photometry • Calculating WFC3 Zeropoints with STSynphot • WFC3/UVIS Pixel Area Map Corrections for Subarrays

[FDauphin]

@mackjenn Thanks for the suggestion! Updated the README via #64.