docs: add note about using npx cli command (#114)

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<!-- What changes are being made? (What feature/bug is being fixed here?) -->
**What**: Add a couple of sentences about running the from the command line with `npx` - they're almost verbetim the "heads up" that @kentcdodds suggested in https://github.com/jfmengels/all-contributors-cli/issues/92#issuecomment-391766876..

<!-- Why are these changes necessary? -->
**Why**: I was having difficulty understanding how to run the commands and I found this text in issue #92 super helpful - I figured it would be for others too 😄 

<!-- How were these changes implemented? -->
**How**: Updated text in README

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**Checklist**:
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- [x] Documentation
- [ ] Tests -- NA
- [x] Ready to be merged <!-- In your opinion, is this ready to be merged as soon as it's reviewed? -->
- [ ] Added myself to contributors table -- ironically I'm not sure how to do this 🤣 <!-- this is optional, see the contributing guidelines for instructions -->

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README.md

@@ -81,6 +81,10 @@ npm run contributors:generate
 
 ## Usage
 
+A quick note: We recommend that you install `all-contributors-cli` as a dependency in your project.
+If you do that then you can run the `all-contributors` binary from within your `package.json` scripts, or you can run it in your terminal with `npx all-contributors`.
+Below we'll just show `all-contributors` to keep things simple, but if you're having any difficulties, then give the `npx all-contributors-cli` route a try :smiley_cat:
+
 ### Generating the contributors list
 
 Please add following placeholders in [`files`](#configuration) to specify the generation area first.