Create README.md

This creates a README.md with a very basic description of what the
project is about and intended use example.

README.md

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+# WebAssembly System Interface Implementation for Web Browsers
+
+This module provides an implementation of the WebAssembly System Interface for Web Browsers
+
+## Supported Syscalls
+
+### wasi_snapshot_preview1
+
+- [ ] args_get
+- [ ] args_sizes_get
+- [ ] environ_get
+- [ ] environ_sizes_get
+- [ ] clock_res_get
+- [ ] clock_time_get
+- [ ] fd_advise
+- [ ] fd_allocate
+- [ ] fd_close
+- [ ] fd_datasync
+- [ ] fd_fdstat_get
+- [ ] fd_fdstat_set_flags
+- [ ] fd_fdstat_set_rights
+- [ ] fd_filestat_get
+- [ ] fd_filestat_set_size
+- [ ] fd_filestat_set_times
+- [ ] fd_pread
+- [ ] fd_prestat_get
+- [ ] fd_prestat_dir_name
+- [ ] fd_pwrite
+- [ ] fd_read
+- [ ] fd_readdir
+- [ ] fd_renumber
+- [ ] fd_seek
+- [ ] fd_sync
+- [ ] fd_tell
+- [ ] fd_write
+- [ ] path_create_directory
+- [ ] path_filestat_get
+- [ ] path_filestat_set_times
+- [ ] path_link
+- [ ] path_open
+- [ ] path_readlink
+- [ ] path_remove_directory
+- [ ] path_rename
+- [ ] path_symlink
+- [ ] path_unlink_file
+- [ ] poll_oneoff
+- [ ] proc_exit
+- [ ] proc_raise
+- [ ] sched_yield
+- [ ] random_get
+- [ ] sock_recv
+- [ ] sock_send
+- [ ] sock_shutdown
+
+## Usage
+
+```javascript
+import Context from "./wasi_snapshot_preview1.js";
+
+const context = new Context({
+});
+
+const instance = await WebAssembly.instantiateStreaming(fetch("module.wasm"), {
+  wasi_snapshot_preview1: context.exports,
+});
+
+context.memory = instance.exports.memory;
+
+if (module.exports._start) {
+  instance.exports._start();
+} else if (module.exports._initialize) {
+  instance.exports._initialize();
+} else {
+  throw new Error("No entry point found");
+}
+```