Add manual for netmap on FreeBSD

BUILDING_FREEBSD_KERNEL_FOR_NETMAP.md

@@ -0,0 +1,35 @@
+Installing netmap in FreeBSD
+
+Try to build kernel module for current kernel:
+```bash
+cd /usr/src/sys/modules/netmap
+make 
+make install
+kldload netmap
+```
+
+But you could hit this bug:
+```bash
+KLD netmap.ko: depends on kernel - not available or version mismatch
+linker_load_file: Unsupported file type
+```
+
+And should rebuild kernel manually.
+
+Install SVN:
+```bash
+pkg install devel/subversion
+```
+
+Download base repository for FreeBSD 10 stable (replace 10 by your FreeBSD version):
+```svn checkput https://svn0.ru.freebsd.org/base/stable/10 /usr/src```
+
+Build and install new kernel:
+```bash
+cd /usr/src/sys/amd64/conf
+cp GENERIC KERNELWITHNETMAP
+cd /usr/src
+make buildkernel KERNCONF=KERNELWITHNETMAP
+make installkernel KERNCONF=KERNELWITHNETMAP
+```
+