Organize landing page (#1769)

The landing page (`README.md`) has been growing larger and less tidy over time as we try to cram more and more information into it.
This change makes a few edits to try to make the landing page shorter and more to the point:

* Streamline the opening lines and try to make them focus on the credibility of the system

* Break off the list of major features into its own subsection and try to highlight the ones that our current users say they benefit from the most

* Move a lot of the information about documentation, examples, Shader Playground, etc. into their own sub-pages to avoid clutter

* Break out the list of dependencies in the `License` section to make sure we are being accurate

With this change the landing page links to the User's Guide directly, so we probably need to get that rendering nicely ASAP.

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+Slang Documentation
+===================
+
+This directory contains documentation for the Slang system.
+Some of the documentation is intended for users of the language and compiler, while other documentation is intended for developers contributing to the project.
+
+Getting Started
+---------------
+
+The Slang [User's Guide](https://shader-slang.github.io/slang/user-guide/) provides an introduction to the Slang language and its major features.
+
+The [API user's guide](api-users-guide.md) gives information on how to drive Slang programmatically from an application.
+There is also documentation specific to using the [`slangc`](command-line-slangc.md) command-line tool.
+
+Advanced Users
+--------------
+
+For the benefit of advanced users we provide detailed documentation on how Slang compiles code for specific platforms.
+The [target compatibility guide](target-compatibility.md) gives an overview of feature compatibility for targets. 
+
+The [CPU target guide](cpu-target.md) gives information on compiling Slang or C++ source into shared libraries/executables or functions that can be directly executed. It also covers how to generate C++ code from Slang source.  
+
+The [CUDA target guide](cuda-target.md) provides information on compiling Slang/HLSL or CUDA source. Slang can compile to equivalent CUDA source, as well as to PTX via the nvrtc CUDA complier.
+
+Contributors
+------------
+
+For contributors to the Slang project, the information under the [`design/`](design/) diretory may help explain the rationale behind certain design decisions and help when ramping up in the codebase.
+
+Research
+--------
+
+The Slang project is based on a long history of research work. While understanding this research is not necessary for working with Slang, it may be instructive for understanding the big-picture goals of the language, as well as why certain critical decisions were made.
+
+A [paper](http://graphics.cs.cmu.edu/projects/slang/) on the Slang system was accepted into SIGGRAPH 2018, and it provides an overview of the language and the compiler implementation.
+Yong He's [dissertation](http://graphics.cs.cmu.edu/projects/renderergenerator/yong_he_thesis.pdf) provided more detailed discussion of the design of the Slang system.