Documentation update (#148)

Author: wzheng

README.md

@@ -8,11 +8,12 @@ Opaque is a package for Apache Spark SQL that enables encryption for DataFrames
 
 This project is based on the following NSDI 2017 paper [1]. The oblivious execution mode is not included in this release.
 
-This is an alpha preview of Opaque, which means the software is still in development (not production-ready!). It currently has the following limitations:
+This is an alpha preview of Opaque, but the software is still in active development. It currently has the following limitations:
 
 - Unlike the Spark cluster, the master must be run within a trusted environment (e.g., on the client).
 
-- Not all Spark SQL operations are supported. UDFs must be [implemented in C++](#user-defined-functions-udfs).
+- Not all Spark SQL operations are supported (see the [list of supported operations](#supported-functionalities)).
+UDFs must be [implemented in C++](#user-defined-functions-udfs).
 
 - Computation integrity verification (section 4.2 of the NSDI paper) is currently work in progress.
 
@@ -136,6 +137,41 @@ Next, run Apache Spark SQL queries with Opaque as follows, assuming [Spark 3.0](
     // | baz|    5|
     // +----+-----+
     ```
+
+## Supported functionalities
+
+This section lists Opaque's supported functionalities, which is a subset of that of Spark SQL. Note that the syntax for these functionalities is the same as Spark SQL -- Opaque simply replaces the execution to work with encrypted data.
+
+### Data types
+Out of the existing [Spark SQL types](https://spark.apache.org/docs/latest/sql-ref-datatypes.html), Opaque supports
+
+- All numeric types except `DecimalType`, which is currently converted into `FloatType`
+- `StringType`
+- `BinaryType`
+- `BooleanType`
+- `TimestampTime`, `DateType`
+- `ArrayType`, `MapType`
+
+### Functions
+We currently support a subset of the Spark SQL functions, including both scalar and aggregate-like functions.
+
+- Scalar functions: `case`, `cast`, `concat`, `contains`, `if`, `in`, `like`, `substring`, `upper`
+- Aggregate functions: `average`, `count`, `first`, `last`, `max`, `min`, `sum`
+
+UDFs are not supported directly, but one can [extend Opaque with additional functions](#user-defined-functions-udfs) by writing it in C++.
+
+
+### Operators
+
+Opaque supports the core SQL operators:
+
+- Projection
+- Filter
+- Global aggregation and grouping aggregation
+- Order by, sort by
+- Inner join
+- Limit
+
     
 ## User-Defined Functions (UDFs)
 
@@ -171,4 +207,4 @@ Now we can port this UDF to Opaque as follows:
 
 ## Contact
 
-If you want to know more about our project or have questions, please contact Wenting ([email protected]) and/or Ankur ([email protected]).
+If you want to know more about our project or have questions, please contact Wenting ([email protected]) and/or Ankur ([email protected]).