Add note about Rosetta 2

An Apple Silicon Mac needs to have Rosetta 2 installed to run x64
code, but it's installed on demand on first use.  If you've never
used anything Intel-based, attempts to run cp2 will fail with "bad
CPU" messages (apparently command-line tools don't know how to
demand Rosetta).

(One trick if you don't have an x64 program sitting around: go into
Applications > Utilities and get info on the Terminal app.  There
will be a checkbox that says "Open using Rosetta".  Check that,
restart Terminal, and you should be asked to install Rosetta.)

See change 80d4300 for why we don't have an Apple Silicon build
(besides it being redundant).

ndocs/top/Install.md

@@ -52,7 +52,10 @@ In the shell, change to the directory where the files were unzipped.  Then:
    to make it executable.  The system adds a "quarantine" flag to anything downloaded from the
    Internet, so you need to do an extra step before you can execute the program.  In the directory
    where the files were unpacked, run `xattr -d com.apple.quarantine *`.  Run `./cp2 version`
-   to confirm this was successful.
+   to confirm this was successful.  The x64 code requires Rosetta 2 to run on Apple Silicon macs;
+   if you've never run an Intel-based program on your Mac, it won't be present, and you will see
+   "bad CPU type in executable" messages.  [Apple support](https://support.apple.com/en-us/102527)
+   has instructions for getting Rosetta 2 installed.
 
 The commands are: