added readme file

Author: maxschwarzmueller

README.md

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+# Course Code & Materials
+
+This repository contains the course source code and other extra materials like slides.
+
+## How to use
+
+The code snapshots are organized in multiple **branches** where every branch **represents a course section**.
+
+For example, the branch [01-getting-started](https://github.com/academind/react-complete-guide-code/tree/01-getting-started) holds all code snapshots and extra materials for section 1 of the course ("Getting Started").
+
+You can switch branches via the branch dropdown above the directory explorer.
+
+![Click on the branch dropdown and then select the appropriate branch for the course section you're looking for](./selecting-a-branch.jpg)
+
+In most branches, you'll find multiple folders which organize the section-specific content further:
+
+- Often, you'll find a `/code` subfolder which contains any relevent code snapshots for the given course section
+- You also often find `/slides` folders which - guess what - contain the slides for the module
+- `/extra-files` typically contains extra files like `.css` files that might be attached to individual lectures in that course module
+
+The folder names should generally be self-explanatory but also feel free to simply click around and see which materials are available.
+
+## Using code snapshots
+
+Code snapshots (which you find in `/code`) are there for you to compare your code to mine and find + fix errors you might have in your code.
+
+You can either view my code directly here on Github (you can open + view code files without issues here) or you download the snapshots.
+
+### Downloading code snapshots
+
+You can download all the content of a branch via the "Code" button here on Github. You can then either [clone](https://docs.github.com/en/github/creating-cloning-and-archiving-repositories/cloning-a-repository) the repository or simply download the selected branch content as a ZIP file.
+
+**Important:** You always download the **entire branch content!**
+
+You can then dive into the interesting folders (e.g. the individual code snapshots) locally on your hard drive.