Yuumi Runes explained
There's one set of runes for Yuumi on patch
11.15
that is the clear favorite for LoL players and pros alike. The best Yuumi Runes on the position 
Support are Sorcery as the primary path and
Precision as the secondary path with a pick rate of 96%. Besides runes you should take a look on Yuumi Build and if you are interested in pro matches take a look at Yuumi Probuilds. If you want to discuss
Yuumi then you should visit the champion subreddit.
Primary path on Yuumi
Sorcery empoweres abilities of Yuumi and increases the available champion resources. Most of high elo players – and our LoL pros in the the major regions – have agreed that
Summon Aery is the best keystone rune to run on
Yuumi. This keystone rune adds more poke damage to your abilities. And additionally grants an extra shield, whenever Yuumi empowers or protects an ally with abilities. This is usually followed up by taking
Manaflow Band,
Transcendence and
Gathering Storm.
This is usually followed up by taking
Manaflow Band for additional mana and mana regenaration.
Transcendence for some more Ability Haste.
Gathering Storm increases your late game power.
Secondary path on Yuumi
When it comes to your secondary path, most
Yuumi players prefer to go with
Precision.
Precision improves attack and sustained damage on Yuumi troughout the match. On the
Precision path, the best runes for Yuumi are
Presence of Mind and
Cut Down.
Presence of Mind helps Yuumi with the mana problems by restoring missing mana, if a champion is killed. If the enemy team consists of multiple tanks, then the rune
Cut Down is the best choice.