Adding getting started information, how to build and generate modules

docs/getting-started.md

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+Welcome to AzureStack Powershell repository!
+
+The repo holds the modules for the AzureStack Hub operator persona. The Azurestack hub admin APIs are exposed as powershell via this repo
+
+Most of the modules are generated with the autorest tool (https://github.com/Azure/autorest.powershell) and some modules are hand written.
+
+## Contents
+### /src folder
+This is for source of all the admin api modules. This could also contain the helper cmdlets over azurestack/azure integration
+For many of the modules only the metadata configuration files would be present as the modules are generated as described in the following sections
+
+### /tools folder
+This contains files that are essential for devops pipelines and security scans
+
+### /docs folder
+This contains the migration guides and general information docs
+
+## Dev Setup
+### Prerequisites
+1. Install NodeJs: https://nodejs.org/ (recommended v10, https://nodejs.org/download/release/v10.15.3/)
+    a. Please do make sure that you have the recommended version. The latest seems to have issues in generation
+2. install new autorest beta packge: npm install -g @autorest/autorest
+3. Install PS core: npm install -g pwsh
+4. Install .net core: npm install -g dotnet-sdk-2.2
+
+### Build
+
+1. Go to module directory, e.g. src/Azs.Network.Admin
+2. Run: 
+    * autorest-beta
+    * pwsh
+    * .\build-module.ps1
+3. All generated cmdlets will be inside 'exports' folder
+4. The module can be imported using the 'Azs.<ModuleName>.Admin.psd1' (e.g. Azs.Network.Admin.psd1) from the module directory
+
+### Customizations:
+The docs for cusotmization can be found under https://github.com/Azure/autorest.powershell/tree/master/docs
+* Input specs files: declare it inside the readme.md file in the azure-rest-api-specs  repo using input-file cmd
+e.g. 
+``` yaml
+input-file:
+    - "$(this-folder)/Microsoft.Network.Admin/preview/2015-06-15/Network.json"
+    - "$(this-folder)/Microsoft.Network.Admin/preview/2015-06-15/LoadBalancers.json"
+    - "$(this-folder)/Microsoft.Network.Admin/preview/2015-06-15/PublicIpAddresses.json" 
+    - "$(this-folder)/Microsoft.Network.Admin/preview/2015-06-15/Quotas.json"
+    - "$(this-folder)/Microsoft.Network.Admin/preview/2015-06-15/VirtualNetworks.json"
+```
+* Include the above file in the azure-powershell module directory's readme.md file using require cmd 
+
+``` yaml 
+require:
+  - $(repo)/specification/azsadmin/resource-manager/network/readme.md
+```
+$(repo) and other common settings are defined in the [file](https://github.com/Azure/azurestack-powershell/blob/master/src/readme.azurestack.md)
+* Directives:
+Built-in Directives support renaming cmdlets/parameters/properties of models, adding alias, setting default value, etc. These are declared in readme.md and would take effect for the generated module. Refer to https://github.com/Azure/autorest/blob/master/docs/powershell/directives.md for the supported directives.
+Example: https://github.com/Azure/azure-powershell/blob/stackadmin/src/StackAdmin/Azs.Network.Admin/readme.md
+    i. Removing InputObject parameter from the New-* cmdlet
+        ## variant removal (parameter InputObject) from all New cmdlets -- parameter sets CreateViaIdentity and CreateViaIdentityExpanded
+  - where:
+      verb: New
+      variant: ^CreateViaIdentity(.*)
+     remove: true
+
+* Customization:
+For other customizations that directives do not support, use customization instead. Refer to https://github.com/Azure/autorest/blob/master/docs/powershell/customization.md for full instruction.
+A simple example is to change the param type from String to SecureString, to achieve this:
+    1. Hide the cmdlet in md so that the original param with String type won't conflict with the SecureString param with same name;
+    2. Generate the module and copy the .ps1 file from internal folder to custom folder;
+    3. Edit the copied .ps1 file to do the customization. Change the param type to SecureString, and convert to plaintext inside cmdlet, replace the PSBoundParameters with the converted value;
+    4. Call the internal cmdlet with the updated PSBoundParameters https://github.com/azure/azurestack-powershell/blob/master/src/Azs.Backup.Admin/custom/Set-AzsBackupConfiguration.ps1
+
+