Suggestions for improvement - mdBulkEdit.py

@MarvinPerry
Nice jobs on the scripts - very useful. I went through `mdBulkEdit.py` and made some notes / suggestions that could help future users:

**Documentation improvement**
- As this repo grows and more scripts are introduced, it would be helpful for the scripts to have their own folders with their own README. The main README would be a landing page that directs users to the individual script folders.
- Step by step instructions for running the script and what to expect would be very useful.
- Users _must_ download python3 from Anaconda, as this is what the ArcGIS Python API requires. Regular python3 doesn't work.
- Users can either clone the repo or copy the script of interest into a text editor and save it in their specified directory.
- Users must have navigated to the directory with the script in order to run it.
- `` is case sensitive.
- This script will only pull items that are shared publicly.
- Currently only works with ISO 19139 Metadata Implementation Specification (administrators need to set this as their default metadata standard in their org settings).
- It would be helpful to know what the script will generate -- metaDataTable.csv and an XML file for each item.
- When having to run the `-c` script a second time, the message to the user should explain why.

For the CSV file:
- Which columns in the CSV file should the user _not_ edit? `mdFileID`?
- `resTitle` should be the first column
- Why don't we include all fields from the metadata standard in the CSV?
- A key is required for users to understand what the fields are, or the fields should be human readable / match the XML online.
- Certain fields require certain types of input, eg date fields need to be in date format.

**Script improvement/extensions**
- Work on all publicly shared services regardless of license.
- Work for all metadata standards on AGOL (FGDC CSDGM, INSPIRE, ISO 19139 GML3.2, North American Profile of ISO19115 2003).
- A user can point the script to only access a particular folder within their ArcGIS Online content.
- A user can point the script to only access a particular group within their ArcGIS Online organization.