some formatting on top-level README

README.md

@@ -1,6 +1,7 @@
 # CCNA 200-301 Notes
 
-Personal notes for learning [CCNA](https://www.cisco.com/site/us/en/learn/training-certifications/certifications/enterprise/ccna/index.html) topics
+Personal notes for learning [CCNA](https://www.cisco.com/site/us/en/learn/training-certifications/certifications/enterprise/ccna/index.html) topics.
+
 
 ## Contents
 * [Network Devices](./contents/network_devices/)
@@ -9,11 +10,14 @@ Personal notes for learning [CCNA](https://www.cisco.com/site/us/en/learn/traini
 * [Intro to Cisco IOS CLI](./contents/intro_to_cisco_cli/)
 * [Ethernet LAN Switching](./contents/ethernet_lan_switching/)
 
+
 ## Network node icons
+
 The following are the icons use in some diagrams from [Jeremy's IT Lab](https://www.youtube.com/playlist?list=PLxbwE86jKRgMpuZuLBivzlM8s2Dk5lXBQ) I used in this repo:
 
-![icon_glossary](/docs/node_icons.png)
+![Icon used in various diagrams and examples](/docs/node_icons.png)
+
 
 ## Sources
+
 * [Jeremy's IT Lab](https://www.youtube.com/playlist?list=PLxbwE86jKRgMpuZuLBivzlM8s2Dk5lXBQ)
-* [Cisco CCNA Study Materials](https://learningnetwork.cisco.com/s/article/200-301-ccna-study-materials)