chore: add route filtering doc

data/filter.txt

@@ -0,0 +1,24 @@
+# For FRR Rules:
+#   cat filter.txt | \
+#       grep -e  ^[0-9] | \
+#       awk '{ print "ip prefix-list nx3-in seq " $1 " " $2 " " $3 " ge " $4 " le " $5}' | \
+#       sed "s_/\([0-9]\+\) ge \1_/\1_g;s_/\([0-9]\+\) le \1_/\1_g"
+#
+# For BIRD Rules:
+#   cat filter.txt | \
+#       awk 'BEGIN {printf "function is_valid_network() {\n return net ~ [\n" } \
+#            /^[0-9]/ && $2 ~ /permit/ {printf "     %s{%s,%s},\n", $3, $4, $5};' | \
+#       sed "$ s/,$/\n  ];\n}/"
+
+# The rules MUST be sorted by the number column first and then the first matching rule MUST be used.
+# ROAs MUST be checked against these rules and max-length of the ROA NUST NOT be longer than allowed by the matching rule.
+
+#Nr   Action  Prefix           MinLen  MaxLen  # Comment
+
+1001  permit  172.24.0.0/24    28      32      # dn42 anycast range
+1002  permit  172.25.0.0/24    28      32      # dn42 anycast range
+1003  permit  172.26.0.0/24    28      32      # dn42 anycast range
+1004  permit  172.27.0.0/24    28      32      # dn42 anycast range
+1100  permit  172.24.0.0/14    21      29      # dn42 main net
+
+9999  deny    0.0.0.0/0        0       32      # block the rest

data/filter6.txt

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+# To FRR Rules:
+#   cat filter6.txt | \
+#       grep -e  ^[0-9] | \
+#       awk '{ print "ipv prefix-list nx3v6-in seq " $1 " " $2 " " $3 " ge " $4 " le " $5}' | \
+#       sed "s_/\([0-9]\+\) ge \1_/\1_g;s_/\([0-9]\+\) le \1_/\1_g"
+#
+# For BIRD Rules:
+#   cat filter6.txt | \
+#       awk 'BEGIN {printf "function is_valid_network() {\n return net ~ [\n" } \
+#            /^[0-9]/ && $2 ~ /permit/ {printf "     %s{%s,%s},\n", $3, $4, $5};' | \
+#       sed "$ s/,$/\n  ];\n}/"
+
+# The rules MUST be sorted by the number column first and then the first matching rule MUST be used.
+# ROAs MUST be checked against these rules and max-length of the ROA NUST NOT be longer than allowed by the matching rule.
+
+# Nr    Action    Prefix                      MinLen  MaxLen    # Comment
+
+1001    permit    fd00::/8                    44       64       # ULA (defined)
+
+9999    deny      ::/0                         0      128       # block the rest