Add 0.18.0 release date to maintenance-table.md

Author: fanquake

_includes/posts/maintenance-table.md

@@ -8,5 +8,5 @@
 | 0.15 | 2017-09-15 | 2018-10-03  | 2019-08-01 |
 | 0.16 | 2018-02-26 | after v0.18 | TBA        |
 | 0.17 | 2018-10-03 | after v0.19 | TBA        |
-| 0.18 | TBA*       | after v0.20 | TBA        |
+| 0.18 | 2019-05-02 | after v0.20 | TBA        |
 | 0.19 | TBA*       | after v0.21 | TBA        |