Update README with full instructions to regenarate both swagger tool and client

Author: sbrossie

README.md

@@ -53,23 +53,46 @@ This client code was generated by the [go-swagger tool](https://github.com/go-sw
 
 We use a modified generator and templates to generate the client, and the [sources are here](https://github.com/killbill/go-swagger).
 
-To generate:
+As explained in the [commit](https://github.com/killbill/go-swagger/commit/4d48bffe307c6043daccf5144b132c55cc783803), we are stuck
+on an older version of `go-swagger`, i.e our fork is not up to date with parent. In addition to this, there is an [issue](https://github.com/go-swagger/go-swagger/issues/2215) compiling
+the tool with recent version of go, and so we are aldo currently stuck using go <= 1.13 -- for building the tool itself, i.e the swagger generator.
+
+To download/install GO 1.13, follow the steps from [here](https://golang.org/doc/manage-install), or simply:
+
+```bash
+# This will install the tool under  ~/sdk by default
+go get golang.org/dl/go1.13
+go1.13 download
+
+# You can then set GOROOT=~/sdk/go1.13 in your shell or inside Goland 's preference (for the project)
+```
+
+To build the (custom) swagger generator
 
 ```bash
-# Update swagger.json
-curl http://localhost:8080/swagger.json | jq "." >swagger.json
+
+# Pre-requisite (needed if template changes)
+brew install go-bindata
 
 # Install swagger tool
 mkdir -p $GOPATH/src/github.com/go-swagger
 cd $GOPATH/src/github.com/go-swagger
 git clone git@github.com:killbill/go-swagger.git
 
-# Required each time the templates change
-go install github.com/go-swagger/go-swagger/cmd/swagger
+# Each time the templates change, following 2 steps are needed:
+
+#1. Regenerate the (template) byte code inside the go-swagger code itself ->  generator/bindata.go should be modified
+go1.13 generate ./generator
+
+#2. build the binary
+go1.13 build cmd/swagger/swagger.go && cp swagger cp ./swagger $GOPATH/bin/swagger
+```
+
+Finally, in order to generate the killbill client (using the custom `swagger` generator):
 
-# You must be in $GOPATH/src to regenerate the files, consider creating a symlink in
-#   ln -s /path/to/killbill/go-swagger go-swagger
-cd $GOPATH/src/github.com/killbill/kbcli
+```bash
+# Update swagger.json
+curl http://localhost:8080/swagger.json | jq "." >swagger.json
 
 # Regenerate the tool
 swagger generate client -f swagger.json -m kbmodel -c kbclient --default-scheme=http