Add Relational Databases Content (#1034)

* Add Relational Databases Content

* Update readme.md

Co-authored-by: Kamran Ahmed <kamranahmed.se@gmail.com>

content/roadmaps/101-backend/content/106-relational-databases/readme.md

@@ -1 +1,10 @@
-# Relational databases
+# Relational Databases
+
+A relational database is **a type of database that stores and provides access to data points that are related to one another**. Relational databases store data in a series of tables. Interconnections between the tables are specified as foreign keys. A foreign key is a unique reference from one row in a relational table to another row in a table, which can be the same table but is most commonly a different table.
+
+<ResourceGroupTitle>Free Content</ResourceGroupTitle>
+<BadgeLink colorScheme='yellow' badgeText='Read' href='https://www.ibm.com/cloud/learn/relational-databases'>Relational Databases</BadgeLink>
+<BadgeLink colorScheme='yellow' badgeText='Read' href='https://learnsql.com/blog/codd-article-databases/'>51 Years of Relational Databases</BadgeLink>
+<BadgeLink badgeText='Watch' href='https://www.edx.org/course/databases-5-sql'>Databases and SQL</BadgeLink>
+<BadgeLink badgeText='Watch' href='https://www.udacity.com/course/intro-to-relational-databases--ud197'>Intro To Relational Databases</BadgeLink>
+<BadgeLink badgeText='Watch' href='https://youtu.be/OqjJjpjDRLc'>What is Relational Database</BadgeLink>