doc: fix incorrect README references

[jouho]

Resolved issues:

We should fix the following items.

• remove the reference to 6,000 lines of code
• remove the reference to “unsupported session renegotiation”

Description of changes:

• Removed "6,000 lines of code" reference. I could put updated number but the number will be changing so I'm voting to simply remove it.
• Fixed documentations around session renegotiation. My understanding is that session renegotiation is now supported for s2n-tls clients, but not for s2n-tls servers based on this comment:
  

  s2n-tls/api/unstable/renegotiate.h

  Line 163 in 9f4baec

  * @note Calling this method on a server connection will fail. s2n-tls servers do not support renegotiation.

Call-outs:

Testing:

This is a documentation update.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.