Add instructions for how to use with Go modules (#431)

* Add instructions on how to use with Go modules

* Update README.md

* Update README.md

* Update README.md

* Update README.md

* Added module info and rewrote some of the content.

README.md

@@ -135,6 +135,43 @@ go get github.com/Azure/go-autorest/autorest/date
 go get github.com/Azure/go-autorest/autorest/to
 ```
 
+### Using with Go Modules
+In [v12.0.1](https://github.com/Azure/go-autorest/pull/386), this repository introduced the following modules.
+
+- autorest/adal
+- autorest/azure/auth
+- autorest/azure/cli
+- autorest/date
+- autorest/mocks
+- autorest/to
+- autorest/validation
+- autorest
+- logger
+- tracing
+
+Tagging cumulative SDK releases as a whole (e.g. `v12.3.0`) is still enabled to support consumers of this repo that have not yet migrated to modules.
+
+If you're using modules and your code directly and/or indirectly depends on previous versions of `go-autorest` (e.g. as a dependency from azure-sdk-for-go), and if you get an ambiguous import for any packages from `go-autorest` you might need to use a replacement to tell Go to use the latest version of the SDK from this repo.
+
+```
+module <your_module_here>
+
+go 1.12
+
+require (
+	github.com/Azure/azure-sdk-for-go v31.1.0+incompatible
+	github.com/Azure/go-autorest/autorest v0.5.0
+	...
+)
+
+replace (
+	github.com/Azure/go-autorest => github.com/Azure/go-autorest v12.3.0+incompatible
+	...
+)
+```
+
+Once azure-sdk-for-go has been converted to modules this replacement should no longer be necessary.
+
 ## License
 
 See LICENSE file.