doc: add a Golang output plugin how-to

Signed-off-by: Eduardo Silva <eduardo@treasure-data.com>

GOLANG_OUTPUT_PLUGIN.md

@@ -0,0 +1,94 @@
+# Fluent Bit + Golang output plugins
+
+The current development version of Fluent Bit, integrates support to load _shared_ plugins built in Golang. The interface still needs some adjustment but is functional, the expectation is to release Fluent Bit v0.10 with fully support for Go.
+
+## Getting Started
+
+Compile Fluent Bit with Golang support, e.g:
+
+```
+$ cd build/
+$ cmake -DFLB_DEBUG=On -DFLB_PROXY_GO=On ../
+$ make
+```
+
+once compiled, we can see a new option in the binary _-e_ which stands for _external plugin_, e.g:
+
+```
+$ bin/fluent-bit -h
+Usage: fluent-bit [OPTION]
+
+Available Options
+  -c  --config=FILE	specify an optional configuration file
+  -d, --daemon		run Fluent Bit in background mode
+  -f, --flush=SECONDS	flush timeout in seconds (default: 5)
+  -i, --input=INPUT	set an input
+  -m, --match=MATCH	set plugin match, same as '-p match=abc'
+  -o, --output=OUTPUT	set an output
+  -p, --prop="A=B"	set plugin configuration property
+  -e, --plugin=FILE	load an external plugin (shared lib)
+  ...
+```
+
+## Build a plugin in Go
+
+To make easier to build plugins, we have published a _fluent-bit-go_ package:
+
+http://github.com/fluent/fluent-bit-go
+
+In the same repository we have published a plugin example called __out_gstdout__ that when runs, it just print the records to the standard output:
+
+https://github.com/fluent/fluent-bit-go/tree/master/examples/out_gstdout
+
+As minimum, a Go plugin looks like this:
+
+```go
+package main
+
+import "github.com/fluent/fluent-bit-go/output"
+
+//export FLBPluginInit
+func FLBPluginInit(ctx unsafe.Pointer) int {
+	return output.FLBPluginRegister(ctx, "gstdout", "Stdout GO!")
+}
+
+//export FLBPluginFlush
+func FLBPluginFlush(data unsafe.Pointer, length C.int, tag *C.char) int {
+    // do something with the data
+    return 0
+}
+
+//export FLBPluginExit
+func FLBPluginExit() int {
+	return 0
+}
+
+func main() {
+}
+```
+
+the code above is a template to write an output plugin, it's really important to keep the package name as  _main_ and add an explicit _main()_ function. This is a requirement as the code will be build as a shared library.
+
+To build the code above, use the following line:
+
+```go
+$ go build -buildmode=c-shared -o out_gstdout.so out_gstdout.go
+```
+
+after a few seconds a shared library called _out\_gstdout.so_ will be available. It's really important to double check the final .so file is what do we expect. Doing a _ldd_ over the library we should see something similar to this:
+
+```
+$ ldd out_gstdout.so
+	linux-vdso.so.1 =>  (0x00007fff561dd000)
+	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007fc4aeef0000)
+	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fc4aeb27000)
+	/lib64/ld-linux-x86-64.so.2 (0x000055751a4fd000)
+```
+
+## Run Fluent Bit + the new plugin
+
+```
+$ bin/fluent-bit -e /path/to/out_gstdout.so -i cpu -o gstdout
+```
+
+for more details or assistance write to eduardo@treasure-data.com