docs: first pass at an updated readme

Author: mrodrig

README.md

@@ -30,12 +30,13 @@ $ npm install @mrodrig/json-2-csv-cli
 
 ## Upgrading?
 
-Upgrading to v3 from v2? Check out the [upgrade guide](https://github.com/mrodrig/json-2-csv/blob/master/upgrade_guides/UPGRADE_2_to_3.md).
+Upgrading to v4 from v3? Check out the [upgrade guide](https://github.com/mrodrig/json-2-csv/blob/master/upgrade_guides/UPGRADE_3_to_4.md).
 
 ## Usage
 
 ```javascript
 let converter = require('json-2-csv');
+const csv = await converter.json2csv(data, options);
 ```
 or
 ```javascript
@@ -45,11 +46,9 @@ Looking for examples? Check out the Wiki: [json-2-csv Wiki](https://github.com/m
 
 ### API
 
-#### `converter.json2csv(array, callback, options)`
+#### `json2csv(array, options)`
 
 * `array` - An array of JSON documents to be converted to CSV.
-* `callback` - A function of the form `function (err, csv)`; 
-  * This function will receive any errors and/or the string of CSV generated.
 * `options` - (Optional) A JSON document specifying any of the following key value pairs:
   * `checkSchemaDifferences` - Boolean - Should all documents have the same schema?
     * Default: `false`
@@ -146,14 +145,9 @@ Looking for examples? Check out the Wiki: [json-2-csv Wiki](https://github.com/m
 
 For examples, please refer to the [json2csv API Documentation (Link)](https://github.com/mrodrig/json-2-csv/wiki/json2csv-Documentation)
 
-#### Promisified Version: `converter.json2csvAsync(array, options)`
-
-Available in version `2.2.0`, this functionality makes use of promises from the `bluebird` module.
-
-#### `converter.csv2json(csv, callback, options)`
+#### `csv2json(csv, options)`
 
 * `csv` - A string of CSV
-* `callback` - A function of the form `function (err, array)`; This function will receive any errors and/or the array of JSON documents generated.
 * `options` - (Optional) A JSON document specifying any of the following key value pairs:
   * `delimiter` - Document - Specifies the different types of delimiters
     * `field` - String - Field Delimiter. 
@@ -181,10 +175,6 @@ Available in version `2.2.0`, this functionality makes use of promises from the
 
 For examples, please refer to the [csv2json API Documentation (Link)](https://github.com/mrodrig/json-2-csv/wiki/csv2json-Documentation)
 
-#### Promisified Version: `csv2jsonAsync(csv, options)`
-
-Available in version `2.2.0`, this functionality makes use of promises from the `bluebird` module.
-
 ### CLI
 Note: As of `3.5.8`, the command line interface functionality has been pulled out to a separate package. Please be sure to
 install the `@mrodrig/json-2-csv-cli` NPM package if you wish to use the CLI functionality shown below:
@@ -244,14 +234,6 @@ To see test coverage, please run:
 $ npm run coverage
 ```
 
-Current Coverage is:
-```
-Statements   : 100% ( 286/286 )
-Branches     : 100% ( 166/166 )
-Functions    : 100% ( 73/73 )
-Lines        : 100% ( 280/280 )
-```
-
 ## Frequently Asked Questions (FAQ)
 Please find the updated list (relocated to the Wiki) here: [Frequently Asked Questions (Link)](https://github.com/mrodrig/json-2-csv/wiki/FAQ)