Improve the ReadMe for the repo
Improve the ReadMe for the repo

1. More emphasis on Microsoft.CodeAnalysis.FxCopAnalyzers package as the primary analyzer package of the repo.
2. Add pointers to docs.microsoft.com documentation for migration from legacy FxCop to FxCop analyzers.

First step towards addressing #2025.
dotnet/roslyn-analyzers,mavasani/roslyn-analyzers,pakdev/roslyn-analyzers,pakdev/roslyn-analyzers,mavasani/roslyn-analyzers,dotnet/roslyn-analyzers