Added summary documentation for STTP data sets

Author: ritchiecarroll

src/README.txt src/README.mdsrc/README.txt renamed to src/README.md

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+# STTP Data Sets
+
+A [data set](https://github.com/sttp/cppapi/blob/master/src/lib/data/DataSet.h) represents an in-memory cache of records that is structured similarly to information defined in a database. The data set object consists of a collection of [data table](https://github.com/sttp/cppapi/blob/master/src/lib/data/DataTable.h) objects.
+
+Data tables define of collection of [data columns](https://github.com/sttp/cppapi/blob/master/src/lib/data/DataColumn.h#L60) where each data column defines a name and [data type](https://github.com/sttp/cppapi/blob/master/src/lib/data/DataColumn.h#L32). Data columns can also be a computed where its value would be derived from other columns and [functions](https://github.com/sttp/cppapi/blob/master/doc/FilterExpressions.md#filter-expression-functions) defined in an expression.
+
+Data tables also define a set of [data rows](https://github.com/sttp/cppapi/blob/master/src/lib/data/DataRow.h) where each data row defines a record of information with a field value for each defined data column. Each field value can by `null` regardless of the defined data column type.
+
+A data set schema and associated records can be read and written data from XML documents. The XML specification used for serialization is the standard [W3C XML Schema Definition Language (XSD)](https://www.w3.org/TR/xmlschema/). See the [ReadXml and WriteXml](https://github.com/sttp/cppapi/blob/master/src/lib/data/DataSet.h#L80) functions.
+
+> :information_source: The STTP data set functionality is modeled after, and generally interoperable with, the [.NET DataSet](https://docs.microsoft.com/en-us/dotnet/api/system.data.dataset). Serialized XML schemas and data saved from a .NET DataSet can be successfully parsed from an STTP data set and vice versa. Note that STTP requires that the schema be included with serialized XML data sets, see [XmlWriteMode.WriteSchema](https://docs.microsoft.com/en-us/dotnet/api/system.data.xmlwritemode). STTP does not attempt to infer a schema from the data. Interoperability with .NET XML schemas also includes [DataColumn expression](https://docs.microsoft.com/en-us/dotnet/api/system.data.datacolumn.expression) functionality, however, STTP defines more [functions](https://github.com/sttp/cppapi/blob/master/doc/FilterExpressions.md#filter-expression-functions) than the .NET implementation, so a serialized STTP data set that includes column expressions using functions not available to a .NET DataColumn will fail to evaluate when accessed from within .NET.