Brief document on decompile command (#998)

* Create decompiling.md

* Update decompiling.md

* Update README.md

README.md

@@ -32,6 +32,8 @@ Once the tooling is installed, you can start the [bicep tutorial](./docs/tutoria
 
 Alternatively, you can try the [Bicep Playground](https://aka.ms/bicepdemo).
 
+If you have an existing ARM Template or set of resources that you would like to convert to `.bicep` format, see [Decompiling an ARM Template](./docs/decompiling.md).
+
 ## How does Bicep work?
 
 First, author your Bicep code using the Bicep language service as part of the [Bicep VS Code extension](./docs/installing.md#bicep-vs-code-extension), then compile that code into an ARM template using the [Bicep CLI](./docs/installing.md#bicep-cli):

docs/decompiling.md

@@ -0,0 +1,36 @@
+# Decompiling an ARM Template
+
+> Requires Bicep CLI v0.2.59 or later
+ 
+The Bicep CLI provides the ability to decompile any existing ARM Template to a `.bicep` file, using the `decompile` command:
+```sh
+bicep decompile "path/to/file.json"
+```
+
+You can use this command to get to a starting point for Bicep authoring. Note that because there is no guaranteed conversion from JSON to Bicep, decompilation may fail, or you may be left with errors/warnings in the generated Bicep file to fix up. See [Limitations](#limiations) for some details of what is not currently possible.
+
+## Exporting a resource group
+You can pass an exported template directly to the `bicep decompile` command to effectively export a resource group to a `.bicep` file.
+
+### Azure CLI
+The following will create a file named 'main.bicep' in the current directory:
+```sh
+az group export --name "your_resource_group_name" > main.json
+bicep decompile main.json
+```
+### Azure PowerShell
+The following will create a file named 'main.bicep' in the current directory:
+```powershell
+Export-AzResourceGroup -ResourceGroupName "your_resource_group_name" -Path ./main.json
+bicep decompile main.json
+```
+
+### Azure Portal
+See [Export Template](https://aka.ms/armexport) for guidance. Use `bicep decompile <filename>` on the downloaded file.
+
+
+## Current Limitations
+The following are temporary limiations on the `bicep decompile` command:
+* Templates using copy loops or conditionals cannot be decompiled
+* Nested templates cannot be decompiled
+* Cross-scope linked templates cannot be decompiled