Extended Documentation on custom functions

[suesas]

the newest upgrades to amplify introduced more capabilities for creating lambda functions. This change might enable many projects to be more streamlined. Previously ive used a custom lambda layer to install sharp, which is a popular image processing library. With the introduction of custom functions I seek to directly install the library during the building process. the documentation contains information on how to install a python or go based lambda, however I would appreciate further documentation on how to setup a Typescript lambda. I figure a Typescript lambda with a customized building process would still be a broadly requested use case.