Readme appears incomplete and hard to trace Documentation, Messages and Codes

[bwilliams1]

For the ErrorProne.Net.CoreAnalyzers, the readme describes about 4 different rules. When installing and running the analyzer it shows 12 different Codes.

There should be one section for each Code/Message or at least a mapping that makes it easy to align them to the related documentation.

1. ERP031
2. EPC13
3. EPC12,EPC12WithoutSuggestion
4. EPC18
5. EPC11,EPC11WithoutSuggestion
6. ERP022
7. EPC19
8. EPC17
9. ERP021
10. EPC15,EPC15WithoutSuggestion
11. EPC14,EPC14WithoutSuggestion
12. EPC16