Documentation: Describe how to deal with snooping https proxies

Disabling SSL verification is far from optimal, but depending on the
circumstances may be the most practical way, so describe how to do
that instead of leaving users confused.

It's also not _that_ bad because git's hashing scheme should uncover
most attempts to tamper with code, either when checking signed tags
or when people push (and see lots of modified commits).

State the command in a way that isn't conductive to careless
copy & paste.

Change-Id: Idbd52ba5d6e8b0f0e891fca16e4159ccef10771a
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37599
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

Documentation/tutorial/part2.md

@@ -58,6 +58,20 @@ the password, and add the following to your `$HOME/.netrc` file:
 where YourUserNameHere is your username, and YourPasswordHere is the password you
 just generated.
 
+If your system is behind a snooping HTTPS proxy, you might also have to
+make its SSL certificate known to curl, a system specific operation.
+If that's not possible for some reason, you can also disable SSL
+certificate verification in git:
+
+	git config [--global] http.sslVerify [true|false]
+
+The `--global` argument sets it for all git transfers of your local
+user, `false` means not to validate the certificate.
+
+If that still doesn't allow you to pull or push changes to the server, the
+proxy is likely tampering with the data stream, in which case there's nothing
+we can do.
+
 ## Part 3: Clone coreboot and configure it for submitting patches
 
 On Gerrit, click on the **Browse** tab in the upper left corner and select