Docs: need to remind users that they have to install the driver packages for their supported database

[Aaronontheweb]

Please describe what you are trying to understand

I ran into this while working on https://github.com/Aaronontheweb/AkkaPersistenceSqlMemoryLeak

With a package setup that looks like:

<Project>
  <PropertyGroup>
    <ManagePackageVersionsCentrally>true</ManagePackageVersionsCentrally>
  </PropertyGroup>
  <ItemGroup Label="App">
    <PackageVersion Include="Akka.Hosting" Version="1.5.37" />
    <PackageVersion Include="Akka.Persistence.Sql.Hosting" Version="1.5.37" />
    <PackageVersion Include="Microsoft.Extensions.Hosting" Version="8.0.0" />
  </ItemGroup>
</Project>

You will get the following runtime exception at startup:

If you do not explicitly add the System.Data.SqlClient (for SQL Server - the package will be different for SQLite / Postgres / etc)

<Project>
  <PropertyGroup>
    <ManagePackageVersionsCentrally>true</ManagePackageVersionsCentrally>
  </PropertyGroup>
  <ItemGroup Label="App">
    <PackageVersion Include="Akka.Hosting" Version="1.5.37" />
    <PackageVersion Include="Akka.Persistence.Sql.Hosting" Version="1.5.37" />
    <PackageVersion Include="Microsoft.Extensions.Hosting" Version="8.0.0" />
    <PackageVersion Include="System.Data.SqlClient" Version="4.9.0" />
  </ItemGroup>
</Project>

We should document this in README.md