Update README.md

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>

README.md

@@ -9,10 +9,59 @@ Very, very alpha:
 1. Limited parts of the language can be parsed.
 2. Limited programs can be evaluated.
 
+As an example of what can be parsed and evaluated, here is a fairly complex P4 program from our unit tests:
+
+```P4
+parser main_parser() {
+    state start {
+        bool where_to = true;
+        string where_from = "here";
+        string where_where = "here";
+        if (where_to) {
+            bool where_from = true;
+            if (where_from) {
+                where_to = false;
+            }
+        }
+        where_from = "there";
+        transition select (where_to) {
+            false: reject;
+            true: accept;
+        };
+    }
+};
+```
+
 Please check back often!
 
 ### Building
 
+#### Requirements And Basic Build
+
+This project uses [code item macros (`CodeItemMacros`)](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0397-freestanding-declaration-macros.md#future-directions) which
+are an experimental feature in Swift and not [_available in production_](https://github.com/swiftlang/swift/blob/1ba5ae105876b15914688cdf8431fd390651296e/include/swift/Basic/Features.def).
+
+Therefore, to compile this project, you must be using a non-production version of the compiler.
+
+With that caveat noted, building can be done with `swift build`:
+
+```console
+$ swift build
+```
+
+#### Testing
+
+To run the P4RSE tests:
+
+```console
+$ swift test
+```
+
+To run the parser tests, from the `tree-sitter-p4` directory:
+```console
+$ npx tree-sitter test
+```
+
 #### Generating Documentation
 
 To build the documentation: