Dedicated README for lambda_http

[ymwjbxxq]

Issue #, if available:
#498

Description of changes:
The README that we add to lambda_http when we publish the crate is the same as we use for the runtime. This readme is too generic to be useful to users. We should create a new README specific for lambda_http that talks about the features in this crate, and how to use it, instead of the generic runtime.

By submitting this pull request

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I confirm that I've made a best effort attempt to update all relevant documentation.

[calavera]

Thanks for creating this new readme, I really appreciate it. In order to use it when we publish new versions, you need to update lambda-http/Cargo.toml replacing the location of the readme file.

[calavera]

There is just one missing mention to Lambda Function URLs. I think it'll be good to go when that's addressed.