docs: Update README.md with grammatical fixes (#4583)

Update the doc for clarity and consistency with present tense. Also,
"rules' description and help properties as well as each checks'
metadata.messages property" → should be "rule's description and help
properties, as well as each check's metadata.messages property"
(singular possessive for "rule" and "check," and add a comma before "as
well as").

locales/README.md

@@ -2,8 +2,8 @@
 
 We welcome any localization for axe-core. For details on how to contribute, see the [Contributing section](../README.md#contributing) of the main README. For details on the message syntax, see [Check Message Template](../doc/check-message-template.md).
 
-To create a new translation for axe, start by running `grunt translate --lang=<langcode>`. This will create a json file with the default English text in it for you to translate. Alternatively, you could copy `_template.json`.
+To create a new translation for axe, start by running `grunt translate --lang=<langcode>`. This will create a JSON file with the default English text in it for you to translate. Alternatively, you could copy `_template.json`.
 
-To update an existing translation file, re-run `grunt translate --lang=<langcode>`. This will add new messages used in English and remove messages which were not used in English.
+To update an existing translation file, re-run `grunt translate --lang=<langcode>`. This will add new messages used in English and remove messages that are no longer used in English.
 
-`_template.json` is a generated file which is created every time axe is built. It's compiled using each rules' `description` and `help` properties as well as each checks' `metadata.messages` property. To update the `_template.json` file you'll need to update the corresponding [rule](../lib/rules) or [check](../lib/checks) metadata file and rebuild.
+`_template.json` is a generated file which is created every time axe is built. It's compiled using each rule's `description` and `help` properties, as well as each check's `metadata.messages` property. To update the `_template.json` file you'll need to update the corresponding [rule](../lib/rules) or [check](../lib/checks) metadata file and rebuild.