usingcurl/ipfs: add

Fixes #327

usingcurl.md

@@ -28,4 +28,5 @@ here to help you do your file transfer tasks as conveniently as possible.
   * [TELNET](usingcurl/telnet.md)
   * [DICT](usingcurl/dict.md)
   * [TLS](usingcurl/tls.md)
+  * [IPFS](usingcurl/ipfs.md)
   * [Copy as curl](usingcurl/copyas.md)

usingcurl/ipfs.md

@@ -0,0 +1,22 @@
+# IPFS
+
+IPFS is the *InterPlanetary File System*. curl supports IPFS only via a *HTTP
+gateway*. It means that it understands IPFS URLs when given to it, but you
+must also provide a working gateway URL for curl to use to retrieve the
+content. curl does not speak IPFS natively.
+
+## Gateway
+
+The `--ipfs-gateway` lets the user specify the IPFS HTTP gateway URL. Like this:
+
+    curl --ipfs-gateway http://localhost:8080 ipfs://bafybeigagd5nmnn2iys2f3d/
+
+If you opt to go for a remote gateway you should be aware that you completely
+trust the gateway. This is fine in local gateways as you host it yourself.
+With remote gateways there could potentially be a malicious actor returning
+you data that does not match the request you made, inspect or even interfere
+with the request. You will not notice this when getting IPFS using curl.
+
+If the `--ipfs-gateway` option is not used, curl will check the `IPFS_GATEWAY`
+environment variable for guidance and if not set, the `~/.ipfs/gateway` file
+that can be used to identify the gateway.