Explain why master key is 4096 bits

Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>

protecting-code-integrity.md

@@ -221,9 +221,11 @@ from other PGP users you come across.
 
 If you are using Linux, you should already have GnuPG installed.  On a Mac,
 you should install [GPG-Suite](https://gpgtools.org) or you can use `brew
-install gnupg2`. On a Windows PC, you should install [GPG4Win](https://www.gpg4win.org).
-For all other platforms, you'll need to do your own research
-to find the correct places to download and install GnuPG.
+install gnupg2`. On a Windows PC, you should install
+[GPG4Win](https://www.gpg4win.org), and you will probably need to adjust some
+of the commands in the guide to work for you, unless you have a unix-like
+environment set up. For all other platforms, you'll need to do your own
+research to find the correct places to download and install GnuPG.
 
 ##### GnuPG 1 vs. 2
 
@@ -334,6 +336,13 @@ possible to start using ED25519 keys right now, it is likely that you will
 come across tools and hardware devices that will not be able to handle them
 correctly.
 
+You may also wonder why the master key is 4096-bit, if later in the guide we
+state that 2048-bit keys should be good enough for the lifetime of RSA public
+key cryptography. The reasons are mostly social and not technical: master keys
+happen to be the most visible ones on the keychain, and some of the developers
+you interact with will inevitably judge you negatively if your master key has
+fewer bits than theirs.
+
 #### Generate the master key
 
 To generate your new master key, issue the following command, putting in the