Add README

README.md

+# `ghc-wasm-meta`
+
+This repo provides convenient methods of using x86_64-linux binary
+artifacts of GHC's wasm backend.
+
+## Getting started as a nix flake
+
+This repo is a nix flake. The default output is a derivation that
+bundles all provided tools:
+
+```sh
+$ nix shell https://gitlab.haskell.org/ghc/ghc-wasm-meta/-/archive/master/ghc-wasm-meta-master.tar.gz
+$ echo 'main = putStrLn "hello world"' > hello.hs
+$ wasm32-wasi-ghc hello.hs -o hello.wasm
+[1 of 2] Compiling Main             ( hello.hs, hello.o )
+[2 of 2] Linking hello.wasm
+$ wasm-run ./hello.wasm
+hello world
+```
+
+First start will download a bunch of stuff, but won't take too long
+since it just patches the binaries and performs little real
+compilation. There is no need to set up a binary cache.
+
+## Getting started without nix
+
+For Ubuntu 20.04 and similar glibc-based distros, this repo provides a
+`setup.sh` script that installs the provided tools to `~/.ghc-wasm`:
+
+```sh
+$ ./setup.sh
+...
+Everything set up in /home/username/.ghc-wasm.
+Run 'source /home/username/.ghc-wasm/env' to add tools to your PATH.
+```
+
+After installing, `~/.ghc-wasm` will contain:
+
+  - `env` which can be sourced into the current shell to add the tools
+    to `PATH`
+  - `add_to_github_path.sh` which can be run in GitHub actions, so
+    later steps in the same job can access the tools from `PATH`
+
+`setup.sh` can be configured via these environment variables:
+
+  - `PREFIX`: installation destination, defaults to `~/.ghc-wasm`
+  - `BIGNUM_BACKEND`: which `ghc-bignum` backend to use, can be either
+    `gmp` or `native`, defaults to `gmp`
+  - `SKIP_GHC`: set this to skip installing `cabal` and `ghc`
+
+`setup.sh` requires `cc`, `curl`, `jq`, `unzip` to run.