Makefile: added support for "make swarm" command (#1412)

Makefile

@@ -0,0 +1,11 @@
+# This Makefile is meant to be used by people that do not usually work
+# with Go source code. If you know what GOPATH is then you probably
+# don't need to bother with make.
+
+GOBIN = $(shell pwd)/build/bin
+
+swarm:
+	build/env.sh go run build/ci.go install ./cmd/swarm
+	@echo "Done building."
+	@echo "Run \"$(GOBIN)/swarm\" to launch swarm."
+

README.md

@@ -27,6 +27,16 @@ Swarm is a distributed storage platform and content distribution service, a nati
 
 Building Swarm requires Go (version 1.11 or later).
 
+To simply compile the `swarm` binary without a `GOPATH`:
+
+    $ git clone https://github.com/ethersphere/swarm
+    $ cd swarm
+    $ make swarm
+
+You will find the binary under `./build/bin/swarm`.
+
+To build a vendored `swarm` using `go get` you must have `GOPATH` set. Then run:
+
     go get -d github.com/ethersphere/swarm
 
     go install github.com/ethersphere/swarm/cmd/swarm