Add cloudwatch-alarm-to-slack lambda

[unixorn]

Description

Add link to cloudwatch-alarm-to-slack

Type of changes

• A link to an external resource like a blog post, tutorial or video
• Add/remove a framework
• Add/remove a lambda function
• Add/remove a utility
• Text cleanups/typo fixes

Copyright Assignment

• This list is Apache 2.0 licensed, and I agree that the contribution in this PR is subject to that license. If you do not check this, I cannot add your contribution to the list.

Checklist:

• I have confirmed that the link(s) in my PR work.
• All entries in the PR are single lines (unless they're a framework) and are in the appropriate (functions, frameworks, resources) section.
• New entries have been added in alphabetical order in their section.
• I have read the CONTRIBUTING document.
• All new and existing tests passed.
• I have stripped leading aws- or aws-lambda- from the link name. This makes it easier to find things by name by preventing a big clusters in the A sections of the sublists.