Update README.md / linking

Chapter on linking. If someone updates just with pull or just git
checkout to this branch, the dependencies might not be in place (or
in this particular case you have the CoaP stuff twice there - ending
linking errors).

Author: Janne Kiiskilä

README.md

@@ -409,4 +409,8 @@ This should resolve the issue:
 ```		
 cp configs/eth-wifi-mbedignore ./.mbedignore		
 ```		
- 		
+
+#### Linking problems?
+
+Due to re-structuring of mbed-client-c the CoAP library might exist twice in your directory tree, if you updated to the latest version of this repository. If moving from an older mbedOS version mbedOS 5.4 based release, please run `mbed deploy` to ensure all repos get updated.
+