instruct GitHub to create empty, instead of incomplete, release tarballs (via .gitattributes)

[slayoo]

PURPOSE: instruct GitHub to create empty, instead of incomplete, release tarballs (via .gitattributes)

TYPE: enhancement

KEYWORDS: release, gitmodules

SOURCE: @slayoo

DESCRIPTION OF CHANGES:
Problem:

Solution:
Make the auto-generated files empty (seems that disabling auto-generation entirely is not supported)

ISSUE: none ?

LIST OF MODIFIED FILES: .gitattributes added

TESTS CONDUCTED:
Tested to work in another project: https://github.com/gnudatalanguage/gdl/blob/master/.gitattributes

RELEASE NOTE: minor change not affecting the proper release files, hence not needed to be mentioned in release notes

[islas]

This looks like a good semi-solution to GitHub's lack of archive control on releases! I think ideally we would want to just disable it, but this should make the error obvious to those that download the wrong archive. We should still keep the bold warning in addition to this.

[slayoo]

FWIW, here's a link to github workflow step we use in GDL to upload submodule-aware tarballs to GitHub release pages automatically (when creating a release via GitHub website):
https://github.com/gnudatalanguage/gdl/blob/144aad86a0d5846543c99b77acc193ab02e5adac/.github/workflows/main.yml#L60-L110

[weiwangncar]

The results from regression tests:

Test Type              | Expected  | Received |  Failed
= = = = = = = = = = = = = = = = = = = = = = = =  = = = =
Number of Tests        : 23           24
Number of Builds       : 60           57
Number of Simulations  : 158           150        0
Number of Comparisons  : 95           86        0

Failed Simulations are: 
None
Which comparisons are not bit-for-bit: 
None