docs: 5.0.0 upgrade guide

Author: mrodrig

upgrade_guides/UPGRADE_4_to_5.md

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+# Upgrade Guide - v4 to v5
+
+## Breaking Changes
+
+* Asynchronous methods are now synchronous
+
+Thanks to [@Nokel81](https://github.com/Nokel81) for the pull request to convert the module to convert the asynchronous internal flow to be entirely synchronous. This helps enable certain use cases where a conversion is needed in a synchronous manner, or where asynchronous code might not be feasible. While these changes shift the module towards a more synchronous focused use case, it's still entirely possible to perform JSON to CSV or CSV to JSON conversions for an asynchronous use case too.
+
+```javascript
+const converter = require('json-2-csv');
+
+// Synchronous:
+
+const csv = converter.json2csv([ { level: 'info', message: 'Our first test' }]);
+console.log('First output is', csv);
+
+// Asynchronous:
+
+async function runConversion() {
+    return converter.json2csv([ { level: 'info', message: 'Another test...' }]);
+}
+
+async function runAsync() {
+    const csv = await runConversion();
+    console.log('Second output is', csv);
+}
+
+runAsync();
+console.log('This can run before the second output appears...');
+```
+
+Example output:
+
+```
+First output is level,message
+info,Our first test
+This can run before the second output appears...
+Second output is level,message
+info,Another test...
+```