Add README.md to core, execution and physical-plan crates (#7970)

* Add README.md to `core`, `execution` and `physical-plan` crates

* prettier

* Update datafusion/physical-plan/README.md

* Update datafusion/wasmtest/README.md

---------

Co-authored-by: Daniël Heres <danielheres@gmail.com>

datafusion/core/README.md

@@ -0,0 +1,26 @@
+<!---
+  Licensed to the Apache Software Foundation (ASF) under one
+  or more contributor license agreements.  See the NOTICE file
+  distributed with this work for additional information
+  regarding copyright ownership.  The ASF licenses this file
+  to you under the Apache License, Version 2.0 (the
+  "License"); you may not use this file except in compliance
+  with the License.  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing,
+  software distributed under the License is distributed on an
+  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+  KIND, either express or implied.  See the License for the
+  specific language governing permissions and limitations
+  under the License.
+-->
+
+# DataFusion Common
+
+[DataFusion](df) is an extensible query execution framework, written in Rust, that uses Apache Arrow as its in-memory format.
+
+This crate contains the main entrypoints and high level DataFusion APIs such as SessionContext, and DataFrame and ListingTable.
+
+[df]: https://crates.io/crates/datafusion

datafusion/execution/README.md

@@ -0,0 +1,26 @@
+<!---
+  Licensed to the Apache Software Foundation (ASF) under one
+  or more contributor license agreements.  See the NOTICE file
+  distributed with this work for additional information
+  regarding copyright ownership.  The ASF licenses this file
+  to you under the Apache License, Version 2.0 (the
+  "License"); you may not use this file except in compliance
+  with the License.  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing,
+  software distributed under the License is distributed on an
+  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+  KIND, either express or implied.  See the License for the
+  specific language governing permissions and limitations
+  under the License.
+-->
+
+# DataFusion Common
+
+[DataFusion](df) is an extensible query execution framework, written in Rust, that uses Apache Arrow as its in-memory format.
+
+This crate is a submodule of DataFusion that provides execution runtime such as the memory pools and disk manager.
+
+[df]: https://crates.io/crates/datafusion

datafusion/physical-plan/README.md

@@ -0,0 +1,27 @@
+<!---
+  Licensed to the Apache Software Foundation (ASF) under one
+  or more contributor license agreements.  See the NOTICE file
+  distributed with this work for additional information
+  regarding copyright ownership.  The ASF licenses this file
+  to you under the Apache License, Version 2.0 (the
+  "License"); you may not use this file except in compliance
+  with the License.  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing,
+  software distributed under the License is distributed on an
+  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+  KIND, either express or implied.  See the License for the
+  specific language governing permissions and limitations
+  under the License.
+-->
+
+# DataFusion Common
+
+[DataFusion](df) is an extensible query execution framework, written in Rust, that uses Apache Arrow as its in-memory format.
+
+This crate is a submodule of DataFusion that contains the `ExecutionPlan` trait and the various implementations of that
+trait for built in operators such as filters, projections, joins, aggregations, etc.
+
+[df]: https://crates.io/crates/datafusion

datafusion/wasmtest/README.md

@@ -17,9 +17,16 @@
   under the License.
 -->
 
-## wasmtest
+# DataFusion wasmtest
+
+[DataFusion](df) is an extensible query execution framework, written in Rust, that uses Apache Arrow as its in-memory format.
+
+This crate is a submodule of DataFusion used to verify that various DataFusion crates compile successfully to the
+`wasm32-unknown-unknown` target with wasm-pack.
 
-Library crate to verify that various DataFusion crates compile successfully to the `wasm32-unknown-unknown` target with wasm-pack.
+[df]: https://crates.io/crates/datafusion
+
+## wasmtest
 
 Some of DataFusion's downstream projects compile to WASM to run in the browser. Doing so requires special care that certain library dependencies are not included in DataFusion.