Added maintenance tips

tips.md

+# Tips & Tricks - ISQ Maintenance
+
+Ideas and routines which may help you with ISQ maintenance.
+
+### Stucked tubing
+ * PVG magic: Reverse the flow, mechanical debrits at entrance will simply fall off.
+ * Rainbow flush: If something was soluble, it can often be dissolved again. You just need to get the right solvent, why not to try them all?
+ * Prolonged flushing with small flows can do a lot.
+
+### What went wrong??
+ISQ has a very nice logging system. Look into it. Usually there is a clue.