add libsmime3.so for chromebook install (#196)

Fixes ImportError: libsmime3.so: cannot open shared object file: No such file or directory" in Chromebook

https://forums.ankiweb.net/t/for-cb-users-i-suggest-reading-this-post-to-install-anki-linux-in-cb-instead/40149

src/platform/linux/installing.md

@@ -14,7 +14,7 @@ On recent Ubuntu installations, please use the following before
 installing:
 
 ```shell
-sudo apt install libxcb-xinerama0 libxcb-cursor0
+sudo apt install libxcb-xinerama0 libxcb-cursor0 libnss3
 ```
 
 If Anki fails to start after installing, you may be [missing other libraries](./missing-libraries.md).