Add a README and warning.

Author: basil00

README.md

@@ -0,0 +1,82 @@
+pairgen
+=======
+
+Pairgen is a tool for generating pairs of "similar" Bitcoin addresses.  For
+example, generate a pair of addresses that share the first *80* bits:
+
+    $ pairgen 80
+    ...
+    WIF[1] = ...
+    WIF[2] = ...
+
+    shared = 20chars
+    hash160[1] = 53e1f4f491509f9012bd901be5147447f770018b
+    hash160[2] = 53e1f4f491509f9012bd825ce1e9599b253188ef
+
+    shared = 15chars
+    addr[1] = 18eXmgR5Svoqqa6PaYVrKvbH6hvrp5xe3A
+    addr[2] = 18eXmgR5Svoqqa6JXSMmbNaD4Cs5ThcV1P
+
+Pairgen exploits the so-called [birthday
+attack](http://en.wikipedia.org/wiki/Birthday_attack) to find long (partial)
+collisions.  Pairgen's algorithm is similar to other tools such as
+[MD5CRK](http://en.wikipedia.org/wiki/MD5CRK).
+
+Warnings!
+---------
+
+Pairgen is experimental software.
+
+**You must independently verify all generated addresses before use!**  Never
+send Bitcoins to a generated address before uploading (and verifying) the
+private keys using other wallet software.
+
+Usage
+-----
+
+The basic usage of pairgen is:
+
+    pairgen bits
+
+where *bits* is the number of shared bits.
+
+The advanced usage of pairgen is:
+
+    pairgen [--job=JOB] [--message=MESSAGE] bits [distinguished-bits]
+
+The *job* option tells pairgen to store state in the files *job.secret*,
+*job.public* and *job.work*.  Jobs can be stopped and restarted, which is
+useful for large runs.
+
+The *message* option allows the user to specify the message used for 
+signature generation.
+
+The *distinguished-bits* determines how granular the work is.
+
+Hacking
+-------
+
+* *More work is good*: more work = higher chance of finding a solution
+  (unlike vanitygen).
+* *Reuse work*: The *job* option can be used to find multiple solutions.
+  Restart pairgen (with the same command-line arguments) to find the next
+  solution.  This will reuse work, so finding more solutions becomes easier
+  over time.
+* *Split work*: The *job.public* file can safely be distributed.  You can
+  generate work on multiple machines, and combine the work by concatenating
+  the resulting *job.work* files.
+
+BUGS
+----
+
+The *job.secret* file is not encrypted in any way.
+
+If a worker thread finds a loop with no distinguished point, it will loop
+forever.  Currently pairgen just relies on this being unlikely.
+
+LICENSE
+-------
+
+The new pairgen code is GPLv3.  Pairgen incorporates other code (e.g.
+libsecp256k1) distributed under other compatible open source licenses.
+

pairgen.c

@@ -1508,6 +1508,7 @@ int main(int argc, char **argv)
         putchar(addr1[i]);
     set_bold(false);
     printf("%s\n\n", addr1+share);
+    printf("warning: verify the keys/addresses before use!\n\n");
 
     return 0;
 }