update README wrt documentation

README.md

 This is OPAL >= 1.9.9. Be aware of the fact that the documentation is not
-anymore a seperate project. 
+anymore a separate project.
 
-The documentation is in this repository in the 
-directory 'doc' (https://gitlab.psi.ch/OPAL/src/tree/master/doc). 
-
-On machines 
-with LaTeX the User's Reference Manual can be built in the same directory as the 
-code by configuring cmake using -DENABLE_DOCUMENTATION=1 to enable the compilation.
-
-The manual can then be built by calling 'make doc'. If you want 
-to compile the manual every time you compile the code then set 
--DEXCLUDE_DOC_FROM_ALL=0 .
+The documentation is in the [Wiki](https://gitlab.psi.ch/OPAL/src/wikis/home).
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