Add base file-descriptors document.

design/file-descriptors.md

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+# File Descriptors
+File Descriptors (witx type `fd`) are an abstract way of representing access to a resource. More often than not, they are processes (WIP), sockets (WIP), files, or even the console.
+
+# Null File Descriptor
+A File Descriptor of 0 is a null file descriptor. It's used whenever there is *no* file descriptor. Passing this to a method is a no-no, and receiving one from a method must be handled.
+
+# The Console
+When a program is initiated, `stdout`, `stdin`, and `stderr` (respectively `fd` ids `1`, `2`, and `3`) are opened. To print to these, simply call `fd_write` with the corresponding arguments. An simple "Hello World!" program written in `wat` is shown below:
+
+```wat
+(module
+  ;; TODO: small hello world?
+)
+```
+
+# Files
+Files can be opened and closed with the commands `fd_open` and `fd_close` respectively. These commands will fail if the WASM environment did not give the program permission to execute these.