Document the x509 and asn1 packages

[pav-kv]

The x509 and asn1 are forks of the corresponding standard Go packages.

Their README files need more detail on:

• The reasons why the packages are forked.
• How to maintain them.
• How/when to keep up with the upstream changes.

The latter are important since there is a backlog of x509-related issues:

• Don't error on unsupported leaf public keys #531
• Certificate verifier doesn't reject mismatched signature algorithm #699
• Alternative Endpoint for x509.ParsePKIXPublicKey that allows non-fatal errors #740
• Begin testing against latest golang version #765
• Inconsistent license infomation for asn1 and x509 #884
• x509 Certificate verification with Common Name #897
• Extend certcheck to cope with Android attestation extension #865
• [bug] Darwin not considered a UNIX system? #869

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"Why" in a nutshell: Since there are many implementations of x509 in the ecosystem, with various levels of incompleteness, CT logs need to be permissive of certificates that almost follow the spec.