update README.md

Author: Lotes

README.md

@@ -1,2 +1,24 @@
 # how-to-build-a-lexer
-Practicing compiler construction by building a lexer
+Practicing compiler construction by building a lexer.
+
+See the [documentation folder](/docs) for detailed information. It does not contain code. But it describes the background of the code.
+
+## `Main` project
+
+The `Main` project contains a regular expression parser prompt.
+
+```
+Please enter a regular expression!
+> [a-z][a-z0-9]*
+start: 1
+ends: 0
+0 --[48-57],[97-122]--> 0
+1 --[97-122]--> 0
+
+Please enter a regular...
+...
+```
+
+## License
+
+[MIT](/LICENSE)