Add basic troubleshooting section to the README

README

@@ -13,3 +13,15 @@ You need Vim 7.3 or higher, compiled with python support and ideally, with
 the conceal feature.
 
 See doc/clang_complete.txt for help and license.
+
+#### Troubleshooting
+
+The first step is to check values of 'omnifunc' and 'completefunc' options in
+a C++ buffer where completion doesn't work (the value should be ClangComplete).
+This can be done with the following command: set omnifunc? completefunc?
+
+Output of :messages command after startup could also show something useful in
+case there were problems with plugin initialization.
+
+If everything is fine, next step might be to load only clang_complete plugin
+and see if anything changes.