Explicitly set active mode for PM sensor upon initialization

src/PMS/PMS.h

@@ -5,6 +5,9 @@
 
 #define PMS_FAIL_COUNT_SET_INVALID 3
 
+/**
+ * Known to work with these sensors: Plantower PMS5003, Plantower PMS5003, Cubic PM2009X
+ */
 class PMSBase {
 public:
   bool begin(Stream *stream);