Added a readme.txt about the PPP code.

diff --git a/src/netif/ppp/readme.txt b/src/netif/ppp/readme.txt
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+About the PPP code:
+
+The PPP code is not our "own" code - we just copied it from pppd (http://ppp.samba.org/) and adapted it to lwIP.
+Unfortunately, not many here know their way around it too well. Back in 2009, we took the effort to see which
+version of pppd our code relates to and we're pretty much on 2.3.11 with some bugs from 2.4.x backported.
+
+Aside from simple code adaptions, there are some files that are different, however:
+- chpms.c/.h are named chap_ms.c/.h in the original pppd 2.3.11 sources
+- pap.c/.h are named upap.c/.h in the original pppd 2.3.11 sources
+- randm.c is a random generator not included in the original pppd
+- magic.c does not use the C library's random functions, but uses randm.c instead
+- vj.c/.h is an implementation of the Van Jacobson header compression algorithm adapted to lwIP pbufs,
+  probably copied from one of the vjcompress.c files from pppd.
+- ppp.c, ppp.h and ppp_impl.h contain the adaption from pppd to lwIP. This is the "OS"-dependent part like there
+  is an implementation for linux, xBSD etc. in the pppd sources.
+- ppp_oe.c is Marc Boucher's implementation based on NetBSD's if_pppoe.c
+
+There is of course potential for bugs in it, but when analyzing of reporting bugs, it is strongly encouraged to
+compare the code in question to pppd 2.3.11 (our basis) and newer versions (perhaps it's already fixed?) and to
+share this knowledge with us when reporting a bug.
+