Update Porter README and diagrams

[derekpierre]

Type of PR:

• [ ] Bugfix
• [ ] Feature
• [X] Documentation
• [ ] Other

Required reviews:

• [ ] 1
• [X] 2
• [ ] 3

What this does:

• Update Porter's README to include /cbd_decrypt endpoint.
• Update digrams for TACo

Questions for reviewers:

1. Should we rename /cbd_decrypt endpoint (#42 )
2. I moved /retrieve_cfrags to a PRE specific section in the README (this was PRE reencrypt). /get_ursulas remains at a high-level. Any concerns/thoughts about that?
3. Should we bump Porter's version? It is already 1.0.0 on mainnet. If semver, then we aren't introducing breaking API changes so technically it would be a 1.1.0, although using 2.0.0 would be refreshing for the nucypher 7.0.0 release (#41 ).

Issues fixed/closed:

• Fixes #...

Why it's needed:

Explain how this PR fits in the greater context of the NuCypher Network. E.g., if this PR addresses a nucypher/productdev issue, let reviewers know!

Notes for reviewers:

What should reviewers focus on? Is there a particular commit/function/section of your PR that requires more attention from reviewers?

[codecov-commenter]

Codecov Report

Merging #40 (c7d626c) into development (ed902ac) will not change coverage. The diff coverage is n/a.

@@             Coverage Diff              @@
##           development      #40   +/-   ##
============================================
  Coverage        87.72%   87.72%           
============================================
  Files               18       18           
  Lines              709      709           
============================================
  Hits               622      622           
  Misses              87       87           

Files	Coverage Δ
porter/main.py	87.21% <ø> (ø)