Update release number in Makefile to 1.8.1


git-svn-id: svn://svn.code.sf.net/p/irrlicht/code@4619 dfc29bdd-3216-0410-991c-e03cc46cb475
Developer documentation - adding a release checklist.


git-svn-id: svn://svn.code.sf.net/p/irrlicht/code@4620 dfc29bdd-3216-0410-991c-e03cc46cb475