[Readme] Added instructions to update from original repo.

Readme.md

+
+# Welcome to the Miosix kernel
+
+You can find information on how to configure and use the kernel
+at the following url: http://miosix.org
+
+### Update from original repo
+
+This is a fork of the original fedetft/miosix-kernel repo that was made for the Skyward Experimental Rocketry organisation.
+
+If you wish to pull in the changes that were made in the original kernel repo you should:
+
+```
+git fetch upstream
+git merge upstream/testing
+git push
+```
+
+Note that the testing branch is were normally the latest updates
+are made for the Skyward guys.

Readme.txt

-
-Welcome to the Miosix kernel
-============================
-
-You can find information on how to configure and use the kernel
-at the following url: http://miosix.org
-