Added a downgrading guide doc

downgrading.md

@@ -0,0 +1,38 @@
+## Automatically using `ipfs-update`
+
+If you want to do a downgrade (and not a clean install of an older version) you can revert your repo using the fs-migration tools:
+
+```sh
+$ ipfs-2-to-3 -revert -path=$IPFS_PATH
+```
+
+That should be it. You're all set.
+
+## Manually downgrading IPFS versions
+
+**You should not have to do this manually. Automatically downgrading is the preferred method.**
+
+Manually downgrading your IPFS version (for instance, from `0.4.0` to `0.3.8`) - which would involve keeping your data - is currently **not possible**. You can't use the same repository simultaneously for both versions. Here is how you switch. However, you can reinstall from scratch.
+
+First, go to your repo for IPFS, check out the right branch, and re-install.
+
+```sh
+cd $GOPATH/src/github.com/ipfs/go-ipfs
+git checkout master # (or the version you want)
+go install ./cmd/ipfs
+```
+
+Then to backup your repo:
+
+```sh
+mv ~/.ipfs ~/.ipfs.bak
+```
+
+Finally, run:
+
+```sh
+ipfs init
+ipfs daemon
+```
+
+And you should be all set.