'Building your function' README updates (#543)

* README updates

* Update README.md

README.md

@@ -43,16 +43,26 @@ func main() {
 
 # Building your function
 
-Preparing a binary to deploy to AWS Lambda requires that it is compiled for Linux and placed into a .zip file.
+Preparing a binary to deploy to AWS Lambda requires that it is compiled for Linux and placed into a .zip file. When using the `provided`, `provided.al2`, or `provided.al2023` runtime, the executable within the .zip file should be named `bootstrap`. Lambda's default architecture is `x86_64`, so when cross compiling from a non-x86 environment, the executable should be built with `GOARCH=amd64`. Likewise, if the Lambda function will be [configured to use ARM](https://docs.aws.amazon.com/lambda/latest/dg/foundation-arch.html), the executable should built with `GOARCH=arm64`.
 
-## For developers on Linux and macOS
 ``` shell
-# Remember to build your handler executable for Linux!
-# When using the `provided.al2` runtime, the handler executable should be named `bootstrap`
 GOOS=linux GOARCH=amd64 go build -o bootstrap main.go
 zip lambda-handler.zip bootstrap
 ```
 
+## For developers on Linux
+
+On Linux, the Go compiler's default behavior is to link the output executable to the system libc for some standard library functionality (for example, DNS lookups). If the build environment is using a Linux distribution with a GNU libc version newer than the deployment environment, the application when deployed to Lambda may fail with an error like ``/lib64/libc.so.6: version `GLIBC_X.YZ' not found``. 
+
+Most Go applications do not require linking to the system libc. This behavior can be disabled by using the `CGO_ENABLED` environment variable.
+
+```
+CGO_ENABLED=0 go build -o bootstrap main.go
+zip lambda-handler.zip bootstrap
+```
+
+See [Using CGO](#using-cgo)
+
 ## For developers on Windows
 
 Windows developers may have trouble producing a zip file that marks the binary as executable on Linux. To create a .zip that will work on AWS Lambda, the `build-lambda-zip` tool may be helpful.
@@ -81,6 +91,20 @@ $env:CGO_ENABLED = "0"
 go build -o bootstrap main.go
 ~\Go\Bin\build-lambda-zip.exe -o lambda-handler.zip bootstrap
 ```
+
+## Using CGO
+
+For applications that require CGO, the build environment must be using a GNU libc version installed compatible with the target Lambda runtime. Otherwise, execution may fail with errors like ``/lib64/libc.so.6: version `GLIBC_X.YZ' not found``.
+
+| Lambda runtime  | GLIBC version
+| ----- | ---
+| `provided.al2023` | 2.34
+| `provided.al2` | 2.26
+| `provided` and `go1.x` | 2.17
+
+
+Alternatively, Lambda supports container images as a deployment package alternative to .zip files. For more information, refer to the official documentation for [working with with container images](https://docs.aws.amazon.com/lambda/latest/dg/images-create.html).
+
 # Deploying your functions
 
 To deploy your function, refer to the official documentation for [deploying using the AWS CLI, AWS Cloudformation, and AWS SAM](https://docs.aws.amazon.com/lambda/latest/dg/deploying-lambda-apps.html).