Add notes for the pipe stream adapter

2019/System.IO.Pipelines/README.md

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+# Adapter for System.IO.Pipelines
+
+Status: **Approved with Feedback** | 
+[API Ref](https://github.com/dotnet/corefx/issues/27246) |
+[Video](https://www.youtube.com/watch?v=9TSP4awoHuI)
+
+# Notes
+
+* The proposal is to throw an exception when anybody uses synchronous (blocking)
+  APIs on the returned `Stream`.
+    - The reason being that the blocking APIs don't perform as well.
+    - We should reverse the default so that synchronous APIs work by default but
+      advanced customers can opt out so they get a "slap on the wrist" when the
+      stack accidentally uses synchronous APIs.
+    - Actually, we decided to get rid of the argument so that we can make the
+      method `AsStream` idempotent.
+* Extension methods vs. virtual methods
+    - We'd make `AsStream()` virtual methods on `PipeReader` and `PipeWriter`.
+    - Since we don't have default interface methods yet, we should not expose
+      an extension method to avoid behavior inconsistencies.
+* `AsStream()` should return the same instance when called multiple times on the
+  same instance (will be idempotent)
+* We decided against making them structs so that it's consistent `PipeOptions`
+* Remove constants for defaults
+* Renames
+    - `MinimumSegmentSize` -> `BufferSize`
+* `PipeReader.CopyToAsync()`
+    - Should be virtual
+    - Should return `Task`
+* `PipeWriter.CopyFromAsync()`
+    - Should be virtual
+    - Should return `Task`
+    - Should be `protected internal` because the name is very unusual
+* Consider overriding `Stream.CopyToAsync` to handle the case where the caller
+  passed in a stream that was produced from a `PipeWriter`. This way, you can
+  by-pass the naïve `Stream`-based copy and go straight to the pipeline.