Add link to sonic-platform-common repo; Tweak some wording (#297)

sourcecode.md

@@ -2,37 +2,39 @@
 
 ## Imaging and Building tools
 - https://github.com/Azure/sonic-buildimage
-	- Source to build an ONIE installable image 
+	- Source to build an installable SONiC image
 
 ## SAI, Switch State Service
 - https://github.com/Azure/sonic-swss
-	- Switch State Service - Core component of SONiC which processes network switch data    
+	- Switch State Service - Core component of SONiC which processes network switch data
 - https://github.com/Azure/sonic-swss-common
 	- Switch State Service common library - Common library for Switch State Service
 - https://github.com/opencomputeproject/SAI
 	- Switch Abstraction Interface standard headers
 - https://github.com/Azure/sonic-sairedis
-	- c++ library for interfacing to SAI objects in Redis 
+	- C++ library for interfacing to SAI objects in Redis
 - https://github.com/Azure/sonic-dbsyncd
-	- python Redis common functions for LLDP
+	- Python Redis common functions for LLDP
 - https://github.com/Azure/sonic-py-swsssdk
-	- python switch state service library
+	- Python switch state service library
 - https://github.com/Azure/sonic-quagga
-	- Quagga 
+	- Fork of Quagga Software Routing Suite for use with SONiC
 
-## Monitoring and management tools 
+## Monitoring and management tools
 - https://github.com/Azure/sonic-mgmt
 	- Management and automation code used for build, test and deployment automation
 - https://github.com/Azure/sonic-utilities
 	- Various command line utilities used in SONiC
 - https://github.com/Azure/sonic-snmpagent
-	- a net-snmpd agentx subagent 
+	- A net-snmpd agentx subagent
 
 ## Switch hardware drivers
 - https://github.com/Azure/sonic-linux-kernel
-	- kernel patches for various device drivers
+	- Kernel patches for various device drivers
+- https://github.com/Azure/sonic-platform-common
+	- API for implementing platform-specific functionality in SONiC
 - https://github.com/Azure/sonic-platform-daemons
-	- daemons for controlling platform specific functionality in SONiC
+	- Daemons for controlling platform-specific functionality in SONiC
 - https://github.com/celestica-Inc/sonic-platform-modules-cel
 - https://github.com/edge-core/sonic-platform-modules-accton
 - https://github.com/Azure/sonic-platform-modules-s6000