Add wiki link to About Chewing editor box

[Billy4195]

Because the documentation now is written on GitHub wiki, add the link
to "About Chewing editor" box for user to read the documentation

[coveralls]

Coverage remained the same at 94.915% when pulling 5bd2c5a on Billy4195:issue205 into 1679266 on chewing:master.

[david50407]

How about put the link in the "Help" menu rather than in about dialog?

[Billy4195]

I try to add a new box named 'Documentation' in Help menu, but
it doesn't work as my prediction. Changing ui_ChewingEditor.h can
successfully add 'Documentation', but I don't know how ui_ChewingEditor.h
be created, The file 'ui_ChewingEditor.h' is generated after make

[Billy4195]

@david50407 I have add documentation link in 'Help' menu.
Should I remove the link in about dialog??

[coveralls]

Coverage remained the same at 94.915% when pulling 779bbb4 on Billy4195:issue205 into 1679266 on chewing:master.

[david50407]

I think just leave in help menu, and remove the link from about dialog.

[Billy4195]

I have removed the commit that add documentation link in a dialog.

[coveralls]

Coverage remained the same at 94.915% when pulling 90aca9b on Billy4195:issue205 into 1679266 on chewing:master.