Update INSTALL_Centos.md

Fixed a couple of typos and made a couple of instructions clearer. I believe step 5 (updating SDE) needs updating but couldn't remember the correct steps.
Merge pull request #91 from Nutbolt52/patch-1

Update INSTALL_Centos.md
Update INSTALL_Centos.md

Add version hint
Add v0.8 upgrade instructions
Update UPGRADING.md

Add info about special upgrade instructions.
Version Bump
Merge remote-tracking branch 'upstream/dev' into dev