[DOCS] Document MacOS keymap more prominently

It is very confusing to new Gertty users like me that it seems to
function fine except the search key is a noop on MacOS, and the
current documentation for fixing this is buried in a 250 line
configuration example.

This is just a minor documentation fix to make the possible need
for a custom key mapping on MacOS more prominent.

Change-Id: I8053de3a0a64ebeeec9d2d400766a8debe5c17fa

README.rst

@@ -176,6 +176,18 @@ screen.
 To select text (e.g., to copy to the clipboard), hold Shift while
 selecting the text.
 
+MacOS
+~~~~~
+
+The MacOS terminal blocks ctrl+o, which is the default search key combo in
+Gertty. To fix this, a custom keymap can be used on MacOS which modifies the
+search key combo. For example::
+
+  keymaps:
+    - name: default # MacOS blocks ctrl+o
+      change-search: 'ctrl s'
+      interactive-search: 'ctrl i'
+
 Terminal Integration
 --------------------
 

examples/reference-gertty.yaml

@@ -107,7 +107,7 @@ keymaps:
     diff: 'd'
   - name: custom
     review: ['r', 'R']
-  - name: osx # OS X blocks ctrl+o
+  - name: osx # MacOS blocks ctrl+o
     change-search: 'ctrl s'
 # To specify a sequence of keys, they must be a list of keystrokes
 # within a list of key series.  For example: