Move some docs to docs dir (declutter the top).

docs/INSTALL.md

@@ -0,0 +1,33 @@
+[release page]: https://github.com/kubernetes-sigs/kustomize/releases
+[Go]: https://golang.org
+
+## Installation
+
+On macOS, you can install kustomize with Homebrew package
+manager:
+
+    brew install kustomize
+
+For all operating systems, download a binary from the
+[release page].
+
+Or try this to grab the latest official release
+using the command line:
+
+```
+opsys=linux  # or darwin, or windows
+curl -s https://api.github.com/repos/kubernetes-sigs/kustomize/releases/latest |\
+  grep browser_download |\
+  grep $opsys |\
+  cut -d '"' -f 4 |\
+  xargs curl -O -L
+mv kustomize_*_${opsys}_amd64 kustomize
+chmod u+x kustomize
+```
+
+To install from head with [Go] v1.10.1 or higher:
+
+<!-- @installkustomize @test -->
+```
+go get github.com/kubernetes-sigs/kustomize
+```