README updates

Author: nblumhardt

README.md

@@ -11,12 +11,12 @@ Install the package from NuGet:
 dotnet add package Serilog.Expressions
 ```
 
-The package adds extension methods to Serilog's `Filter` and 
+The package adds extension methods to Serilog's `Filter`, `WriteTo`, and 
 `Enrich` configuration objects, along with an `OutputTemplate`
 type that's compatible with Serilog sinks accepting an
 `ITextFormatter`.
 
-### Filtering
+### Filtering example
 
 _Serilog.Expressions_ adds `ByExcluding()` and `ByIncludingOnly()` 
 overloads to the `Filter` configuration object that accept filter
@@ -70,12 +70,49 @@ this is written as:
   </appSettings>
 ``` 
 
-### Enriching
+## Supported configuration APIs
 
-### Formatting
+_Serilog.Expressions_ adds a number of expression-based overloads and helper methods to the Serilog configuration syntax:
+
+ * `Filter.ByExcluding()`, `Filter.ByIncludingOnly()` - use an expression to filter events passing through the Serilog pipeline
+ * `WriteTo.Conditional()` - use an expression to select the events passed to a particular sink
+ * `Enrich.When()` - conditionally enable an enricher when events match an expression
+ * `Enrich.WithComputed()` - add or modify event properties using an expression
+
+## Formatting
+
+_Serilog.Expressions_ includes the `OutputTemplate` class for text formatting. `OutputTemplate` implements `ITextFormatter`, so
+it works with any text-based Serilog sink:
+
+```csharp
+Log.Logger = new LoggerConfiguration()
+    .WriteTo.Console(new OutputTemplate(
+        "[{@t:HH:mm:ss} {@l:u3} ({SourceContext})] {@m} (first item is {Items[0]})\n{@x}"))
+    .CreateLogger();
+```
+
+Note the use of `{Items[0]}`: "holes" in expression-based output templates can include arbitrary expressions.
 
 ## Language reference
 
+### Built-in properties
+
+The following properties are available in expressions:
+
+ * All first-class properties of the event; no special syntax: `SourceContext` and `Items` are used in the formatting example above
+ * `@t` - the event's timestamp, as a `DateTimeOffset`
+ * `@m` - the rendered message
+ * `@mt` - the raw message template
+ * `@l` - the event's level, as a `LogEventLevel`
+ * `@x` - the exception associated with the event, if any, as an `Exception`
+ * `@p` - a dictionary containing all first-class properties; this supports properties with non-identifier names, for example `@p['snake-case-name']`
+
+### Data types
+
+### Functions
+
+### String manipulation
+
 ## Working with the raw API
 
 The package provides the class `SerilogExpression` in the `Serilog.Expressions` namespace