add Parse Error guide

Author: mtrezza

CONTRIBUTING.md

@@ -111,6 +111,26 @@ For example, allowing public read and write to a class may be useful to simplify
 
 Security checks are added in [SecurityChecks.js](https://github.com/parse-community/parse-server/blob/master/src/SecurityChecks.js).
 
+### Parse Error
+
+Introducing new Parse Errors requires the following steps:
+
+1. Research whether an existing Parse Error already covers the error scenario. Keep in mind that reusing an already existing Parse Error does not allow to distinguish between scenarios in which the same error is thrown, so it may be necessary to add a new and more specific Parse Error, eventhough an more general Parse Error already exists.
+⚠️ Currently (as of Dec. 2020), there are inconsistencies between the Parse Errors documented in the Parse Guides, coded in the Parse JS SDK and coded in Parse Server, therefore research regarding the availability of error codes has to be conducted in all of these sources.
+1. Add the new Parse Error to [/src/ParseError.js](https://github.com/parse-community/Parse-SDK-JS/blob/master/src/ParseError.js) in the Parse JavaScript SDK. This is the primary reference for Parse Errors for the Parse JavaScript SDK and Parse Server.
+1. Create a pull request for the Parse JavaScript SDK including the new Parse Errors. The PR needs to be merged and a new Parse JS SDK version needs to be released.
+1. Change the Parse JS SDK dependency in [package.json](https://github.com/parse-community/parse-server/blob/master/package.json) of Parse Server to the newly released Parse JS SDK version, so that the new Parse Error is recognized by Parse Server.
+1. When throwing the new Parse Error in code, do not hard-code the error code but instead reference the error code from the Parse Error. For example:
+    ```javascript
+    throw new Parse.Error(Parse.Error.EXAMPLE_ERROR_CODE, 'Example error message.');
+    ```
+1. Choose a descriptive error message that provdes more details about the specific error scenario. Different error messages may be used for the same error code. For example:
+    ```javascript
+    throw new Parse.Error(Parse.Error.FILE_SAVE_ERROR, 'The file could not be saved because it exceeded the maximum allowed file size.');
+    throw new Parse.Error(Parse.Error.FILE_SAVE_ERROR, 'The file could not be saved because the file format was incorrect.');
+    ```
+1. Add the new Parse Error to the [docs](https://github.com/parse-community/docs/blob/gh-pages/_includes/common/errors.md).
+
 ## Code of Conduct
 
 This project adheres to the [Contributor Covenant Code of Conduct](https://github.com/parse-community/parse-server/blob/master/CODE_OF_CONDUCT.md). By participating, you are expected to honor this code.