Various README and CONTRIBUTING updates

Close GH Issue 69
Close GH PR 457/RT4113
Some other updates

By Rich Salz, Alessandro Ghedini, Steve Marquess, Collin Anderson

(manual cherry-pick of a2aaf8be7e and
b06935f439)

Reviewed-by: Kurt Roeckx <kurt@openssl.org>

CONTRIBUTING

@@ -0,0 +1,32 @@
+HOW TO CONTRIBUTE TO OpenSSL
+----------------------------
+
+Development is coordinated on the openssl-dev mailing list (see
+http://www.openssl.org for information on subscribing). If you
+would like to submit a patch, send it to rt@openssl.org with
+the string "[PATCH]" in the subject. Please be sure to include a
+textual explanation of what your patch does.
+
+You can also make GitHub pull requests. If you do this, please also send
+mail to rt@openssl.org with a brief description and a link to the PR so
+that we can more easily keep track of it.
+
+If you are unsure as to whether a feature will be useful for the general
+OpenSSL community please discuss it on the openssl-dev mailing list first.
+Someone may be already working on the same thing or there may be a good
+reason as to why that feature isn't implemented.
+
+Patches should be as up to date as possible, preferably relative to the
+current Git or the last snapshot. They should follow our coding style
+(see https://www.openssl.org/policies/codingstyle.html) and compile without
+warnings using the --strict-warnings flag.  OpenSSL compiles on many varied
+platforms: try to ensure you only use portable features.
+
+Our preferred format for changes is "diff -u" output. One method is to use
+"git diff"  Another is to generate it something like this:
+
+# cd openssl-work
+# [your changes]
+# ./Configure dist; make clean
+# cd ..
+# diff -ur openssl-orig openssl-work > mydiffs.patch