docs: give advice on project structure to avoid unnecessary 'load' dependencies

Bazel only downloads external dependencies that are needed for building your target? That's not exactly true.

## Problem
Bazel fetches unneeded dependencies due to load statements.

## Reproduce
Considering the following example:

Repo A is your main repo, which depends on some library in repo B. Repo B depends on a test rule in repo C for testing its library.

In A/BUILD:
```
cc_binary(
name = "bin",
srcs = ["main.cc"],
deps = ["@B//:lib"]
)
```

In B/BUILD:
```
load("@C-dev//:def.bzl", "my_cc_test")

cc_library(
name = "lib",
srcs = ["lib.cc"],
)

my_cc_test(
name = "lib_test",
srcs = ["lib_test.cc"],
deps = [
":lib",
"@C-dev//:test",
],
)
```

Although C is only a dev dependency of B, in order to build `//:bin` A has to declare C as a dependency in WORKSPACE and Bazel will fetch C at build time.

The root cause is `load("@C-dev//:def.bzl", "my_cc_test")` being in the same package as `@B//:lib`, in the loading phase, Bazel has to fetch C to correctly parse the BUILD file.

A full repo case is here: https://github.com/meteorcloudy/my_tests/tree/master/dev_deps_test

##