Drop broken p4/docs symlink and create README.md+URL

There was a symlink to a local file, which would instead work better as
a URL to the bpf-docs repository.

Fixes: #304
Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>

src/cc/frontends/p4/docs/README.md

+# External references
+
+See [p4toEbpf-bcc.pdf](https://github.com/iovisor/bpf-docs/blob/master/p4/p4toEbpf-bcc.pdf)

src/cc/frontends/p4/docs/p4toEbpf-bcc.pdf

-../../../../../../bpf-docs/p4toEbpf-bcc.pdf
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