Add examples readme (#785)

examples/README.md

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+# Examples
+
+The examples here are split into a few categories:
+
+- The shaders folder contain various rust-gpu shaders, and are examples of how to use rust-gpu.
+- The runners folder contains programs that build and execute the shaders in the shaders folder using, for example,
+  Vulkan. These programs are not exactly examples of how to use rust-gpu, as they're rather generic vulkan sample apps,
+  but they do contain some infrastructure examples of how to integrate rust-gpu shaders into a build system (although
+  both aren't the cleanest of examples, as they're also testing some of the more convoluted ways of consuming rust-gpu).
+- Finally, the multibuilder folder is a very short sample app of how to use the `multimodule` feature of `spirv-builder`.