Update doc for OpenSSL::X509::Certificate

[Nimesh-Msys]

I think the following documentation is incomplete and should be updated.

openssl/ext/openssl/ossl_x509cert.c

Lines 733 to 734 in fdcda97

	* raw = File.read "cert.cer" # DER- or PEM-encoded
	* certificate = OpenSSL::X509::Certificate.new raw

DER-encoded certificates are binary and read by using File.binread.
File.read gives the error:

OpenSSL::X509::CertificateError (not enough data)

Hence to avoid confusions, the document should be something like:

  *   raw = File.read "cert.cer" # PEM-encoded 
  *   # For DER-encoded: raw = File.binread "cert.cer"
  *   certificate = OpenSSL::X509::Certificate.new raw 

Or may be a note to inform this.