Added instructions for generating ssl files on macOS

[flavioheleno]

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	no
Related tickets	none
Documentation	if this is a new feature, link to pull request in https://github.com/php-http/documentation that adds relevant documentation
License	MIT

What's in this PR?

This PR adds instructions for generating ssl files on macOS, required to run the library's test suite.

Why?

When trying to generate ssl files on macOS Catalina (10.15.7), openssl fails to read the CA private key (check below), this PR adds instructions on how to workaround that problem in a very straightforward way.

$ ./generate.sh 
Generating a 2048 bit RSA private key
......................+++
............+++
writing new private key to stdout
-----BEGIN ENCRYPTED PRIVATE KEY-----
[REDACTED]
-----END ENCRYPTED PRIVATE KEY-----
-----
Generating RSA private key, 2048 bit long modulus
..........................+++
......+++
e is 65537 (0x10001)
Signature ok
subject=/C=FR/ST=Ile-de-France/L=Paris/O=PHP-HTTP/CN=socket-adapter
Getting CA Private Key
Error opening CA Private Key privkey.pem
4361031276:error:02FFF002:system library:func(4095):No such file or directory:/AppleInternal/BuildRoot/Library/Caches/com.apple.xbs/Sources/libressl/libressl-47.140.1/libressl-2.8/crypto/bio/bss_file.c:255:fopen('privkey.pem', 'r')
4361031276:error:20FFF002:BIO routines:CRYPTO_internal:system lib:/AppleInternal/BuildRoot/Library/Caches/com.apple.xbs/Sources/libressl/libressl-47.140.1/libressl-2.8/crypto/bio/bss_file.c:257:
unable to load CA Private Key

Checklist

• Updated CHANGELOG.md to describe BC breaks / deprecations | new feature | bugfix
• Documentation pull request created (if not simply a bugfix)

^ Have not done any of these as I don't think it applies to this case.