Add documentation for g:qs_hi_priority update

Commit 4d7ef43 allows g:qs_hi_priority
to be set by the user, this adds documentation to the help file and the
README to explain it.

Author: bradford-smith94

README.md

@@ -114,6 +114,13 @@ augroup qs_colors
 augroup END
 ```
 
+The highlight groups are applied using a priority (see: `:help :syn-priority`
+and `:help matchadd()` for more detail). The default priority used is `1` but
+you can override this if needed by setting it yourself using:
+```vim
+let g:qs_hi_priority = 2
+```
+
 ### Toggle highlighting
 Turn the highlighting on and off with a user command:
 ```

doc/quick-scope.txt

@@ -203,6 +203,16 @@ commands.
   augroup END
 <
 
+
+                                                            *g:qs_hi_priority*
+The highlight groups are applied using |matchadd()| with a priority
+(|:syn-priority|). The default priority used is `1` but you can override this
+if needed by setting it yourself using:
+
+>
+  let g:qs_hi_priority = 2
+<
+
 4.3 ACCEPTED CHARACTERS                                    *qs-accepted-chars*
 ------------------------------------------------------------------------------