Update contributing guidelines to say not to open new issues for algorithms

[ChrisO345]

Describe your change:

• Added a markdown header under Contributing for issues.
• Added paragraph to say not to open new issues

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[tianyizheng02]

@cclauss @ChrisO345 We also need to update our issue templates to reflect this. Currently the "feature request" issue template explicitly states that it's fine to open issues for new algorithms.

[cclauss]

Agreed... Let's make those edits.