Some Grammar fixes (#5858)

* Grammar fixes

1- For `Here's a `: The singular verb form ’s does not seem to agree with the plural subject a few common cases where you might want to try something else. Consider changing the verb form.

2- Added `A` before Reasonably : The noun phrase Reasonably good configuration seems to be missing a determiner before it. Consider adding an article. An article (a, an, or the) is a type of determiner. Possessive adjectives (my, his, our), possessive nouns (Joe’s, mother’s), and quantifiers (each, every) are also determiners. Single countable nouns usually require a determiner.

* Update README.md

README.md

@@ -134,7 +134,7 @@ Please refer to the [User Guide](https://facebook.github.io/create-react-app/doc
 
 - **One Dependency:** There is just one build dependency. It uses Webpack, Babel, ESLint, and other amazing projects, but provides a cohesive curated experience on top of them.
 
-- **No Configuration Required:** You don't need to configure anything. Reasonably good configuration of both development and production builds is handled for you so you can focus on writing code.
+- **No Configuration Required:** You don't need to configure anything. A reasonably good configuration of both development and production builds is handled for you so you can focus on writing code.
 
 - **No Lock-In:** You can “eject” to a custom setup at any time. Run a single command, and all the configuration and build dependencies will be moved directly into your project, so you can pick up right where you left off.
 
@@ -163,7 +163,7 @@ Create React App is a great fit for:
 - **Starting new single-page React applications.**
 - **Creating examples** with React for your libraries and components.
 
-Here’s a few common cases where you might want to try something else:
+Here are few common cases where you might want to try something else:
 
 - If you want to **try React** without hundreds of transitive build tool dependencies, consider [using a single HTML file or an online sandbox instead](https://reactjs.org/docs/try-react.html).