Update plugins.json with disclaimer for Fluid

Added disclaimer as installs via Sketch Toolbox are not working atm.

See this issue: buzzfeed/Sketch-Toolbox#61 and this issue: matt-curtis/Fluid-for-Sketch#15

I'm adding a disclaimer rather than removing it entirely to deter others adding it.

plugins.json

@@ -1290,7 +1290,7 @@
         "owner": "swiadek"
     },
     {
-        "description": "Sketch-flavored Auto Layout-like Constraints",
+        "description": "Sketch-flavored Auto Layout-like Constraints [MUST BE INSTALLED MANUALLY - SEE README.]",
         "name": "fluid-for-sketch",
         "owner": "matt-curtis"
     },
@@ -1299,4 +1299,4 @@
         "name": "sketch-link-underline-align",
         "owner": "zholdas"
     }
-]
+]