Add documentation for -publicsign compiler switch

[ericstj]

Issue Title

The csc switch -publicsign is missing documentation.

General

This switch is used to apply a public key without actually signing it, like -delaysign but unlike -delaysign, -publicsign sets a bit in the assembly file that tells the runtime that the file is actually signed.
This switch was added with https://github.com/dotnet/docs/blob/master/docs/csharp/language-reference/compiler-options/listed-by-category.md.

Issues with Existing Topics

We should add the switch here: https://github.com/dotnet/docs/blob/master/docs/csharp/language-reference/compiler-options/listed-by-category.md

Requests for new Topics

1. Tell us where this topic should go in the Table of Contents.

   • Follow the same pattern as is used https://github.com/dotnet/docs/blob/master/docs/csharp/language-reference/compiler-options/delaysign-compiler-option.md.

2. Write an abstract

   • Cribbed from https://github.com/dotnet/docs/blob/master/docs/csharp/language-reference/compiler-options/delaysign-compiler-option.md

   Sometimes called "fake sign" or "OSS sign" public signing is including
   the public key in an output assembly and setting the "signed" flag, but
   not actually signing the assembly with a private key. This is useful for
   open source projects where people want to build assemblies which are
   compatible with the released "fully signed" assemblies, but don't have
   access to the private key used to sign the assemblies. Since almost no
   consumers actually need to check if the assembly is fully signed, these
   publicly built assemblies are useable in almost every scenario that the
   fully signed one would be used in.

3. Fill in an outline

   • Copy -delaysign outline

4. Suggest reviewers

   • @agocke @nguerrera @DnlHarvey

[mairaw]

@ericstj does this apply to VB as well?

[chwarr]

Looks like VB support for public signing was added in commit dotnet/roslyn@4aea4e0, and overall public sign work was tracked by the now-closed dotnet/roslyn#6915