Add README to 2.1 folder explaining where the 2.1 images have gone

2.1/README.md

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+HTTP 301 Moved Permanently
+--------------------------
+
+**Where are the 2.1 aspnet-docker images?**
+
+In 2.1\*, the ASP.NET Core Docker images have migrated to https://github.com/dotnet/dotnet-docker.
+
+_\*Note: ASP.NET Core 2.1 is still in preview._
+
+**How can I upgrade from 1.x/2.0 to 2.1?**
+
+You can upgrade to 2.1 and by changing the repository name in the `FROM` line in your Dockerfile using this mapping:
+
+Current | Upgrade
+--------|--------------
+`microsoft/aspnetcore:1.0`<br>`microsoft/aspnetcore:1.1`<br>`microsoft/aspnetcore:2.0` | `microsoft/dotnet:2.1-aspnetcore-runtime`
+`microsoft/aspnetcore-build:1.0`<br>`microsoft/aspnetcore-build:1.1`<br>`microsoft/aspnetcore-build:2.0` | `microsoft/dotnet:2.1-sdk`
+
+**I was using NodeJS in `microsoft/aspnetcore-build`, but this is missing from `microsoft/dotnet:2.1-sdk`. What should I do?**
+
+You can either install NodeJS by adding a few lines of code to your Dockerfile that download and extract NodeJS, 
+or you can use the multi-stage feature of Docker and the official NodeJS images.
+
+Sample code to install NodeJS on your own:
+
+```Dockerfile
+# set up node
+ENV NODE_VERSION 8.9.4
+ENV NODE_DOWNLOAD_SHA 21fb4690e349f82d708ae766def01d7fec1b085ce1f5ab30d9bda8ee126ca8fc
+RUN curl -SL "https://nodejs.org/dist/v${NODE_VERSION}/node-v${NODE_VERSION}-linux-x64.tar.gz" --output nodejs.tar.gz \
+    && echo "$NODE_DOWNLOAD_SHA nodejs.tar.gz" | sha256sum -c - \
+    && tar -xzf "nodejs.tar.gz" -C /usr/local --strip-components=1 \
+    && rm nodejs.tar.gz \
+    && ln -s /usr/local/bin/node /usr/local/bin/nodejs
+```
+
+Sample code to use multi-arch build
+```Dockerfile
+FROM node:8 AS node-builder
+WORKDIR /src
+COPY . .
+RUN npm run webpack
+
+FROM microsoft/dotnet:2.1-sdk AS dotnet-builder
+WORKDIR /app
+COPY --from=node-builder /src/dist/*.js  ./dist
+```