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ARROW-4839: [C#] Add NuGet package metadata and instructions.

This change adds the necessary changes for creating the `Apache.Arrow` nuget package.

1. I've updated the `.csproj` with what I think is the correct package metadata. Please take a look and give any feedback on what I got wrong.
2. I've added instructions to the README on how to build the NuGet package. By default, this will produce a `-preview` version of the package. You need to explicitly opt into creating a "stable" version by passing in `-p:VersionSuffix=''`.
3. I've enabled strong naming on the library, which is a recommendation according to the [.NET Open-source library guidance](https://docs.microsoft.com/en-us/dotnet/standard/library-guidance/strong-naming).
    - Note that "strong naming" isn't a security feature - it is just about creating a unique identity for the library.

/cc @stephentoub @ericstj @pgovind @chutchinson @wesm

Author: Eric Erhardt <eric.erhardt@microsoft.com>

Closes #3891 from eerhardt/NuGetPackage and squashes the following commits:

8c0f6805 <Eric Erhardt> PR feedback
89f630be <Eric Erhardt> Remove pack call from CI
b3c6ab87 <Eric Erhardt> Respond to PR feedback.
f4bb1938 <Eric Erhardt> fix up prepare script to escape correctly
1566f40a <Eric Erhardt> PR feedback
c7a9d8da <Eric Erhardt> Enable strong naming on the Apache.Arrow C# library.
5c31e1da <Eric Erhardt> ARROW-4839:  Add NuGet package metadata and instructions.