ensure make tools executes before build-ui (#683)

Co-authored-by: Jeff Mitchell <jeffrey.mitchell@gmail.com>

README.md

@@ -99,6 +99,16 @@ UI assets; which will take a few extra minutes.) Once complete, run Boundary in
 
   ```./$GOPATH/bin/boundary dev```
 
+Please note that building the UI requires `go-bindata`; the easiest way to install
+Boundary's dependent tools is to simply run:
+
+  ```make tools```
+
+Without doing so, you may encounter errors while running `make dev`. It is important
+to also note that using `make tools` will install various tools used for Boundary
+development to the normal Go binary directory; this may overwrite or take precedence
+over tools that might already be installed on the system.
+
 #### Specify a UI Commitish at Build Time
 
 By default the UI will be built from a preselected commit ID from [the UI