Update dependency section in documentation

When installing the `protobuf` dependencies on a Debian machine, I ran into the following error:

```
$ make
protoc-c --c_out=. cstore.proto
gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -g -g -O2 -fstack-protector-strong -Wformat -Werror=format-security -I/usr/include/mit-krb5 -fPIC -pie -fno-omit-frame-pointer -fpic --std=c99 -I. -I./ -I/usr/include/postgresql/9.6/server -I/usr/include/postgresql/internal -D_FORTIFY_SOURCE=2 -D_GNU_SOURCE -I/usr/include/libxml2 -I/usr/include/tcl8.6 -c -o cstore.pb-c.o cstore.pb-c.c
In file included from cstore.pb-c.h:7:0,
from cstore.pb-c.c:9:
/usr/include/protobuf-c/protobuf-c.h:183:20: fatal error: assert.h: No such file or directory
#include
^
compilation terminated.
: recipe for target 'cstore.pb-c.o' failed
make: *** [cstore.pb-c.o] Error 1
```

That is, the `make` command was throwing an error when attempting to compile `cstore_fdw`.

That was solved by adding the dependency `libprotoc-dev` to the list. I suggest updating the `README.md` file to reflect that change. If that makes sense, I've submitted a pull request that updates the documentation.