Create README.md

Author: danielgerlag

src/providers/WorkflowCore.Persistence.PostgreSQL/README.md

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+# PostgreSQL Persistence provider for Workflow Core
+
+Provides support to persist workflows running on [Workflow Core](../../README.md) to a PostgreSQL database.
+
+## Installing
+
+Install the NuGet package "WorkflowCore.Persistence.PostgreSQL"
+
+```
+PM> Install-Package WorkflowCore.Persistence.PostgreSQL -Pre
+```
+
+## Usage
+
+Use the .UsePostgreSQL extension method when building your service provider.
+
+```C#
+services.AddWorkflow(x => x.UsePostgreSQL(@"Server=127.0.0.1;Port=5432;Database=workflow;User Id=postgres;Password=password;", true, true));
+```