add some basic gateway documentation

License: MIT
Signed-off-by: Steven Allen <steven@stebalien.com>

docs/gateway.md

+# Gateway
+
+An IPFS Gateway acts as a bridge between traditional web browsers and IPFS.
+Through the gateway, users can browse files and websites stored in IPFS as if
+they were stored in a traditional web server.
+
+By default, go-ipfs nodes run a gateway at `http://127.0.0.1:5001/`.
+
+We also provide a public gateway at `https://ipfs.io`. If you've ever seen a
+link in the form `https://ipfs.io/ipfs/Qm...`, that's being served from *our*
+gateway.
+
+## Configuration
+
+The gateway's configuration options are (briefly) described in the
+[config](https://github.com/ipfs/go-ipfs/blob/master/docs/config.md#gateway)
+documentation.
+
+## Directories
+
+For convenience, the gateway (mostly) acts like a normal web-server when serving
+a directory:
+
+1. If the directory contains an `index.html` file:
+  1. If the path does not end in a `/`, append a `/` and redirect. This helps
+     avoid serving duplicate content from different paths.<sup>&dagger;</sup>
+  2. Otherwise, serve the `index.html` file.
+2. Dynamically build and serve a listing of the contents of the directory.
+
+<sub><sup>&dagger;</sup>This redirect is skipped if the query string contains a
+`go-get=1` parameter. See [PR#3964](https://github.com/ipfs/go-ipfs/pull/3963)
+for details</sub>
+
+## Filenames
+
+When downloading files, browsers will usually guess a file's filename by looking
+at the last component of the path. Unfortunately, when linking *directly* to a
+file (with no containing directory), the final component is just a CID
+(`Qm...`). This isn't exactly user-friendly.
+
+To work around this issue, you can add a `filename=some_filename` parameter to
+your query string to explicitly specify the filename. For example:
+
+> https://ipfs.io/ipfs/QmfM2r8seH2GiRaC4esTjeraXEachRt8ZsSeGaWTPLyMoG?filename=hello_world.txt
+
+## MIME-Types
+
+TODO