plugin_icon_file.md: note about custom plugin repos (IJSDK-737)
plugin_icon_file.md: note about custom plugin repos (IJSDK-737)

artspb/intellij-sdk-docs,artspb/intellij-sdk-docs,JetBrains/intellij-sdk-docs,JetBrains/intellij-sdk-docs,artspb/intellij-sdk-docs,artspb/intellij-sdk-docs,JetBrains/intellij-sdk-docs