Created Attribute Specifications (markdown)

Attribute-Specifications.md

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+Attribute specifications (`AttributeSpec`) describe the storage location of the data typed by a given `Attribute`, and allow attributes to be re-ordered and filtered without generating unexpected behaviour. 
+
+**Code excerpt: resolving the specification of an individual attribute**
+```go
+package main
+
+import (
+	"fmt"
+	"github.com/sjwhitworth/golearn/base"
+)
+
+func main() {
+
+	// Read the CSV file
+	data, err := base.ParseCSVToInstances("iris_headers.csv", true)
+	// Error check
+	if err != nil {
+		panic(fmt.Sprintf("Couldn't load CSV file (error %s)", err))
+	}
+
+	// Resolve an Attribute
+	as, err := data.GetAttribute(base.NewFloatAttribute("Sepal length"))
+	if err != nil {
+		panic(fmt.Sprintf("Couldn't resolve AttributeSpec (error %s)", err))
+	}
+
+	// Print first column
+	asArr := []base.AttributeSpec{as}
+	data.MapOverRows(asArr, func(row [][]byte, i int) (bool, error) {
+		fmt.Println(base.UnpackBytesToFloat(row[0]))
+		return true, nil
+	})
+
+}
+```
+
+For convenience, `base` also provides [`ResolveSomeAttributes`](https://godoc.org/github.com/sjwhitworth/golearn/base#ResolveAttributes), which retrieves a slice of `AttributeSpecs` for the attributes specified.
+
+**Code excerpt: resolving the specifications of all non-class Attributes**
+```go
+package main
+
+import (
+	"fmt"
+	"github.com/sjwhitworth/golearn/base"
+)
+
+func main() {
+
+	// Read the CSV file
+	data, err := base.ParseCSVToInstances("iris_headers.csv", true)
+	// Error check
+	if err != nil {
+		panic(fmt.Sprintf("Couldn't load CSV file (error %s)", err))
+	}
+
+	// Resolve all non-class Attributes
+	asArr := base.ResolveAttributes(data, base.NonClassAttributes(data))
+	// (ResolveAllAttributes gets AttributeSpecs for every attribute)
+
+	// Print non-class data
+	data.MapOverRows(asArr, func(row [][]byte, i int) (bool, error) {
+		for _, b := range row {
+			fmt.Print(base.UnpackBytesToFloat(b), " ")
+		}
+		fmt.Println()
+		return true, nil
+	})
+
+}
+```