Update glossary.mdx

Added 'reinforcement learning' and 'deep reinforcement learning' to the glossary.

units/en/unit1/glossary.mdx

@@ -2,6 +2,14 @@
 
 This is a community-created glossary. Contributions are welcome!
 
+### Reinforcement Learning
+
+A trial-and-error learning paradigm in which an agent learns an optimal policy for taking an action given an observation about the state of its environment, in order to maximize future discounted rewards.
+
+### Deep Reinforcement Learning
+
+Reinforcement learning in which the policy or value function to be trained (i.e. the agent's brain) is instantiated as a deep neural network.
+
 ### Agent
 
 An agent learns to **make decisions by trial and error, with rewards and punishments from the surroundings**.