Add "fsharp" (F#) (#641)

* Add F# to docs

See https://github.com/docker-library/official-images/pull/1201

* Tweak a few bits for markdownfmt and length

fsharp/README-short.txt

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+F# is a multi-paradigm language encompassing functional, imperative, and object-oriented styles

fsharp/content.md

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+# What is F#?
+
+F# (pronounced F sharp) is a strongly typed, multi-paradigm programming language that encompasses functional, imperative, and object-oriented programming techniques. F# is most often used as a cross-platform CLI language, but can also be used to generate JavaScript and GPU code.
+
+> [wikipedia.org/wiki/F Sharp (programming language)](https://en.wikipedia.org/wiki/F_Sharp_%28programming_language%29)
+
+%%LOGO%%
+
+# How to use this image
+
+## Start your application inside this image
+
+The most straightforward way to use this image is to use it both as the build and runtime environment. In your `Dockerfile`, you can write something similar to the following:
+
+```dockerfile
+FROM fsharp:latest
+COPY . /app
+RUN xbuild /app/myproject.sln
+```
+
+This will copy your application source code into the image and use `xbuild` to build it.

fsharp/github-repo

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+https://github.com/fsprojects/docker-fsharp

fsharp/license.md

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+View [the Apache 2.0 license](https://github.com/fsharp/fsharp/blob/d518f91418ef43a61875a5d932147b97fd0f47f3/LICENSE) for the software contained in this image.