Create SECURITY.md

[JoshSuggs1]

Hello and thank you for contributing to dotnet/reactive. Before you proceed by creating a pull request (PR):

Please make sure your contribution is in line with the project's licensing model and
your employment allows you to contribute to open source.

Please sign the Contributor License Agreement in case the CLA bot asks you
for being a new contributor.

Please make sure you target the main branch when creating the PR.

What is the nature of your contribution?

Bugfix

Please describe what and how the bug manifested itself and what kind of fix did you implement.
Practically, you are writing these descriptions for the search engines so others can find the resolution easier.

Please reference the related issue(s) to the original bug report (example: Fixes #X, Resolves #X).
Note that some behaviors are by design and are not bugs. If you are unsure, please open an issue first.

Please include the relevant unit test(s) that verify the fix works.
It is recommended you write the unit test(s) first to see they failed before your fix; however, there is
no need to post a PR to demonstrate this failure stage. Please post in a state where the issue is resolved.

Enhancement

Please consider opening an issue first to discuss the kind and impact of the enhancement first,
if such issue doesn't exist yet. Also please check if there was a consensus about accepting such an enhancement.

Please describe how, what and why the enhancement is implemented.
Practically, you are writing these descriptions for the search engines so others can find the change details easier.

Please reference the related issue(s) to the original issue report (example: Fixes #X, Resolves #X).

Please include the relevant unit test(s) or benchmark(s) that show or verify the enhancement works.

Feature request

We strongly recommend opening an issue first to discuss the nature and impact of the request first.
If such an issue already exists, please check if there was a consensus and approval for accepting a contribution.

Please describe what request is about, how it works in general and what possible corner cases it or the user should consider.
Please include some basic usage examples of the important components and/or overloads.
Practically, you are writing these descriptions for the search engines so others can find out about the new feature easier.

Please reference the related issue(s) to the original issue report (example: Resolves #X).

Please include unit tests that verify the behavior of the new feature. Please make sure the code coverage is reasonably high with the new code.
Depending on the feature, you may want to include actual multi-threaded test cases to verify the correct async behavior, however,
please be mindful of the time such tests take and try to stay under a few seconds.