Moved app-templates to be under sql-big-data-cluster and added general README

samples/features/sql-big-data-cluster/app-templates/README.md

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+# App Templates
+These are the language kernel templates for deployment and consumption of applications deployed into an on premise SQL Server Big Data Cluster.
+
+Each directory listed is a separate language kernel that is supported for app deployment:
+- Python
+- R
+- SSIS
+- Mleap
+
+Each template contains three primary files:
+- spec.yaml
+    - This is the deployment definition given to the cluster
+- run-spec.yaml
+    - This is the app consumption definition given to the VS Code Ext. client
+- app code
+    - Example code for deployment of that particular type of language kernel
+
+These templates are automatically pulled by the mssqlctl CLI and the App Deploy VS Code Extension.  Customers may also download these templates themselves outside of the clients, if desired.
+
+Future language kernels supported for app deployment will be contributed as templates here.