Add document for supported runtime versions

doc/supportedRuntimeVersions.md

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+# Supported Runtime Versions for Oryx
+
+Below are the supported runtime versions for Oryx that can be pulled via Microsoft Container Registry following the format
+`mcr.microsoft.com/oryx/{platform}:{version}`.
+
+_Note_: the `{version}` used as the tag of the runtime image corresponds to the latest minor _or_ patch version supported by Oryx, _i.e._, `node:8`
+corresponds to the latest version of node `8.x.x` supported by Oryx, and `node:8.0` corresponds to the latest version of node `8.0.x` supported.
+
+The full list of supported platform versions for Oryx can be found [here](supportedPlatformVerions.md).
+
+## .NET
+
+The following are supported runtime versions for the image `mcr.microsoft.com/oryx/dotnetcore:{version}`.
+
+- `1.0`
+- `1.1`
+- `2.0`
+- `2.1`
+- `2.2`
+- `3.0`
+- `3.1`
+- `3.2`
+- `5.0`
+- `6.0`
+- `7.0`
+
+## node
+
+The following are supported runtime versions for the image `mcr.microsoft.com/oryx/node:{version}`.
+
+- `4.4`
+- `4.5`
+- `4.8`
+- `6`
+- `6.2`
+- `6.6`
+- `6.9`
+- `6.10`
+- `6.11`
+- `8`
+- `8.0`
+- `8.1`
+- `8.2`
+- `8.8`
+- `8.9`
+- `8.11`
+- `8.12`
+- `8.16`
+- `9.4`
+- `10`
+- `10.1`
+- `10.10`
+- `10.12`
+- `10.14`
+- `10.16`
+- `10.17`
+- `12`
+- `12.7`
+- `12.8`
+- `12.9`
+- `12.12`
+- `12.13`
+- `14`
+- `16`
+
+## PHP
+
+The following are supported runtime versions for the image `mcr.microsoft.com/oryx/php:{version}`.
+
+- `5.6`
+- `7.0`
+- `7.2`
+- `7.3`
+- `7.4`
+- `8.0`
+- `8.1`
+
+## Python
+
+The following are supported runtime versions for the image `mcr.microsoft.com/oryx/python:{version}`.
+
+- `2.7`
+- `3.6`
+- `3.7`
+- `3.8`
+- `3.9`
+- `3.10`
+
+## Ruby
+
+The following are supported runtime versions for the image `mcr.microsoft.com/oryx/ruby:{version}`.
+
+- `2.5`
+- `2.6`
+- `2.7`