fix(doc): add an example)

Author: pascal-blokur

README.md

@@ -30,4 +30,42 @@ It's tedious because the `csv:"Phone"` tag doesn´t match the header
 The goal of this library is to make processing of CSV files with "variable" 
 headers simpler.
 
-## U
+## Usage
+In order to use this library, one needs to have a `csv.Reader` already.
+
+The next requirement is to have a list of the header mappings, as a map.
+These go from the file to the `struct`. 
+In our example above, the header replacemenent would be:
+```go
+headerReplacements := map[string]string{
+	"Nom":    "NAME",
+	"Numéro": "PHONE NUMBER",
+}
+```
+Finally, we can build a replacer with:
+```go
+replacer := csvheaders.NewReplacer(csvReader, headerReplacements)
+```
+This `replacer` can be fed to csv parsing libraries such as `gocsv` 
+or `goflat`:
+
+```go
+file, err := os.Open("phone_directory.csv")
+if err != nil {
+	...
+}
+defer file.Close()
+
+csvReader := csvheaders.NewReplacer(
+	csv.NewReader(file),
+	map[string]string{
+        "Nom": "NAME",
+        "Numéro": "PHONE NUMBER",
+    })
+
+var rows []*Person
+err := gocsv.UnmarshalCSV(csvReader, &rows)
+if err != nil {
+	...
+}
+```