Added a downgrading guide doc

downgrading.md

@@ -0,0 +1,26 @@
+## Downgrading IPFS versions
+
+Downgrading your IPFS version (for instance, from 4.0 to 3.8) is not as complicated as upgrading. You can't use the same repository simultaneously for both versions. Here is how you switch
+
+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.