Add README to Html.Abstractions and update Components README (#27833)

* Add README to Html.Abstractions
* Address feedback from peer review
* Update build instructions for components project
* Apply suggestions from code review
Co-authored-by: Rick Anderson <3605364+Rick-Anderson@users.noreply.github.com>
Co-authored-by: Rick Anderson <3605364+Rick-Anderson@users.noreply.github.com>

src/Components/README.md

@@ -31,7 +31,9 @@ The following contains a description of each sub-directory in the `Components` d
 
 ### Build
 
-To build this specific project from source, you can follow the instructions [on building a subset of the code](https://github.com/dotnet/aspnetcore/blob/master/docs/BuildFromSource.md#building-a-subset-of-the-code).
+To build this specific project from source, follow the instructions [on building a subset of the code](../../BuildFromSource.md#building-a-subset-of-the-code).
+
+**Note:** You also need to run the preceding `build` command in the command line before building in VS to ensure that the Web.JS dependency is built.
 
 ### Test
 
@@ -46,7 +48,7 @@ The E2E tests are located in the top-level `tests` folder in this directory. The
 
 Each app server mounts the same `BasicTestApp` application under each scenario.
 
-To run the tests for this project, you can [run the tests on the command line](https://github.com/dotnet/aspnetcore/blob/master/docs/BuildFromSource.md#running-tests-on-command-line) in this directory.
+To run the tests for this project, [run the tests on the command line](../../BuildFromSource.md#running-tests-on-command-line) in this directory.
 
 ## More Information
 

src/Html/Abstractions/README.md

@@ -0,0 +1,21 @@
+# Html.Abstractions
+
+This project contains types for manipulating HTML content.
+
+## Description
+
+This project contains interfaces and abstractions used in MVC to support writing and modifying HTML content. For a full list of the types defined in this class, see [the namespace documentation](https://docs.microsoft.com/dotnet/api/microsoft.aspnetcore.html).
+
+## Development Setup
+
+### Build
+
+To build this specific project from source, follow the instructions [on building a subset of the code](../../../BuildFromSource.md#building-a-subset-of-the-code).
+
+### Test
+
+To run the tests for this project, [run the tests on the command line](../../../BuildFromSource.md#running-tests-on-command-line) in this directory.
+
+## More Information
+
+For more information, see the [ASP.NET Core README](../../../README.md).