Added swift-log-ecs to README.md

[rwbutler]

I've created a new logging backend for SwiftLog in order to log in ECS Log format. Updated README.md to include the library.

Motivation:

I created the library for any own use with Vapor but hopefully it will be useful for others as well. Can be used independently of Vapor and chained with other LogHandlers.

Modifications:

Updated README.md to include swift-log-ecs in the list of libraries.

Result:

README.md will include swift-log-ecs in the list of libraries.

P.S. If a PR is not the correct way to let you know about new libraries let me know and I'll close the PR and resubmit in the appropriate manner.