docs: refer to alternate shells in CONTRIBUTING.md

Some users may not be using the default shell for their system, and
Linux users are the most likely to do this, though this can be seen in
macOS as well. People will often use shells like fish which is not POSIX
compliant but works very well and has great auto-completion, it's very
user friendly.

The reason why I care about this is that the instructions specify to use
.venv/bin/activate which is a bash script and will not run on fish or
csh, and if you take a look at the script, indeed it says it will only
run with bash.

Python will provide alternate scripts for other shells, and at least on
my system (CachyOS), those are scripts for Powershell, bash, fish and
csh. People using these alternate shells, who don't know this may be
confused when running the script referenced in CONTRIBUTING.md and
seeing it fail completely.

The only real change in this commit was adding under the "Linux/macOS"
command in step 3 (step 2 prior to my last commit) of the "Creating a
Python Virtual Environment" section, a short instruction to switch to
the default shell, or use the appropriate script for the user's
preferred shell if any (Since, at least on my system, there aren't
scripts for every shell, for example there is no activate.zsh file).

CONTRIBUTING.md

@@ -66,6 +66,9 @@ If you wish to launch the source version of TagStudio outside of your IDE:
 -   Windows w/Powershell: `.venv\Scripts\Activate.ps1`
 -   Windows w/Command Prompt: `.venv\Scripts\activate.bat`
 -   Linux/macOS: `source .venv/bin/activate`
+       If you use an alternative shell like fish or csh, you may wish to switch to the default shell (Usually bash) for this project, or alternatively, you can see the other activation scripts left by Python such as:
+       - Fish: `source .venv/bin/activate.fish`
+       - CSH: `source .venv/bin/activate.csh`
 
 4. Install the required packages: