Add missing dependency to installation instructions

[vollkorn1982]

The instructions in INSTALL.md are missing argcomplete as it is listed in the requirements.txt.

Description

I added the missing argcomplete in INSTALL.md.

If argcomplete is not installed on your system, starting gourmet results in an error:

$ gourmet 
Traceback (most recent call last):
  File "/home/jan/.local/bin/gourmet", line 5, in <module>
    from gourmet.GourmetRecipeManager import launch_app
  File "/home/jan/.local/lib/python3.8/site-packages/gourmet/GourmetRecipeManager.py", line 11, in <module>
    from gourmet import (
  File "/home/jan/.local/lib/python3.8/site-packages/gourmet/batchEditor.py", line 3, in <module>
    from . import gglobals
  File "/home/jan/.local/lib/python3.8/site-packages/gourmet/gglobals.py", line 8, in <module>
    from .optionparser import args
  File "/home/jan/.local/lib/python3.8/site-packages/gourmet/optionparser.py", line 2, in <module>
    import argcomplete
ModuleNotFoundError: No module named 'argcomplete'

How Has This Been Tested?

I tried installing gourmet on a fresh Ubuntu 20.04 installation and ran into the error message above. Then I installed the missing dependency by running python3 -m pip install argcomplete and tried running gourmet again. This time gourmet started without a problem.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[cydanil]

Fantastic! Thanks a lot for taking the time to fix the docs :)