Add info about end-to-end testing about docs HOT 5 CLOSED

vpp-agent % make integration-tests --debug
GNU Make 3.81
Copyright (C) 2006  Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.

This program built for i386-apple-darwin11.3.0
Reading makefiles...
Updating goal targets....
 File `integration-tests' does not exist.
Must remake target `integration-tests'.
# running integration tests
VPP_IMG=ligato/vpp-base:20.05 ./tests/integration/vpp_integration.sh
# go.ligato.io/vpp-agent/v3/plugins/linux/nsplugin/linuxcalls
plugins/linux/nsplugin/linuxcalls/namespace_api.go:95:9: cannot use &namedNetNsHandler literal (type *namedNetNsHandler) as type NamedNetNsAPI in return argument:
	*namedNetNsHandler does not implement NamedNetNsAPI (missing CreateNamedNetNs method)
plugins/linux/nsplugin/linuxcalls/system_linuxcalls.go:55:9: undefined: syscall.Mount
make: *** [integration-tests] Error 2

➜  vpp-agent git:(master) remake integration-tests --debug --verbosity=full
GNU Make 4.3+dbg-1.5
Built for x86_64-apple-darwin19.5.0
Copyright (C) 1988-2020 Free Software Foundation, Inc.
Copyright (C) 2015, 2017 Rocky Bernstein.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Reading makefiles...
Reading makefile 'Makefile'...
Reading makefile 'vpp.env' (search path) (no ~ expansion)...
Reading makefile 'scripts/make/buf.make' (search path) (don't care) (no ~ expansion)...
Updating makefiles...
Updating goal targets...
Considering target file 'integration-tests'.
/Users/christophermetz/vpp-agent/Makefile:180 integration-tests
 File 'integration-tests' does not exist.
 Finished prerequisites of target file 'integration-tests'.
Must remake target 'integration-tests'.
# running integration tests
VPP_IMG=ligato/vpp-base:20.05 ./tests/integration/vpp_integration.sh
# go.ligato.io/vpp-agent/v3/plugins/linux/nsplugin/linuxcalls
plugins/linux/nsplugin/linuxcalls/namespace_api.go:95:9: cannot use &namedNetNsHandler literal (type *namedNetNsHandler) as type NamedNetNsAPI in return argument:
	*namedNetNsHandler does not implement NamedNetNsAPI (missing CreateNamedNetNs method)
plugins/linux/nsplugin/linuxcalls/system_linuxcalls.go:55:9: undefined: syscall.Mount
Makefile:181: *** [integration-tests] error 2

#0  integration-tests at /Users/christophermetz/vpp-agent/Makefile:180