Updated to CONTRIBUTING to reflect GitHub, etc.

Reviewed-by: NRichard Levitte <levitte@openssl.org>
Reviewed-by: NTim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5889)

CONTRIBUTING

-HOW TO CONTRIBUTE PATCHES TO OpenSSL
-------------------------------------
+HOW TO CONTRIBUTE TO OpenSSL
+----------------------------
 
 (Please visit https://www.openssl.org/community/getting-started.html for
 other ideas about how to contribute.)
 
-Development is coordinated on the openssl-dev mailing list (see the
-above link or https://mta.openssl.org for information on subscribing).
-If you are unsure as to whether a feature will be useful for the general
-OpenSSL community you might want to 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.
+Development is done on GitHub, https://github.com/openssl/openssl.
 
-To submit a patch, make a pull request on GitHub.  If you think the patch
-could use feedback from the community, please start a thread on openssl-dev
-to discuss it.
+To request new features or report bugs, please open an issue on GitHub
 
-Having addressed the following items before the PR will help make the
-acceptance and review process faster:
+To submit a patch, please open a pull request on GitHub.  If you are thinking
+of making a large contribution, open an issue for it before starting work,
+to get comments from the community.  Someone may be already working on
+the same thing or there may be reasons why that feature isn't implemented.
 
-    1. Anything other than trivial contributions will require a contributor
-    licensing agreement, giving us permission to use your code. See
-    https://www.openssl.org/policies/cla.html for details.
+To make it easier to review and accept your pull request, please follow these
+guidelines:
+
+    1. Anything other than a trivial contribution requires a Contributor
+    License Agreement (CLA), giving us permission to use your code. See
+    https://www.openssl.org/policies/cla.html for details.  If your
+    contribution is too small to require a CLA, put "CLA: trivial" on a
+    line by itself in your commit message body.
 
     2.  All source files should start with the following text (with
     appropriate comment characters at the start of each line and the
@@ -34,21 +34,22 @@ acceptance and review process faster:
         https://www.openssl.org/source/license.html
 
     3.  Patches should be as current as possible; expect to have to rebase
-    often. We do not accept merge commits; You will be asked to remove
-    them before a patch is considered acceptable.
+    often. We do not accept merge commits, you will have to remove them
+    (usually by rebasing) before it will be acceptable.
 
     4.  Patches should follow our coding style (see
-    https://www.openssl.org/policies/codingstyle.html) and compile without
-    warnings. Where gcc or clang is available you should use the
+    https://www.openssl.org/policies/codingstyle.html) and compile
+    without warnings. Where gcc or clang is available you should use the
     --strict-warnings Configure option.  OpenSSL compiles on many varied
-    platforms: try to ensure you only use portable features.
-    Clean builds via Travis and AppVeyor are expected, and done whenever
-    a PR is created or updated.
+    platforms: try to ensure you only use portable features.  Clean builds
+    via Travis and AppVeyor are required, and they are started automatically
+    whenever a PR is created or updated.
 
     5.  When at all possible, patches should include tests. These can
     either be added to an existing test, or completely new.  Please see
     test/README for information on the test framework.
 
     6.  New features or changed functionality must include
-    documentation. Please look at the "pod" files in doc/man[1357]
-    for examples of our style.
+    documentation. Please look at the "pod" files in doc/man[1357] for
+    examples of our style. Run "make doc-nits" to make sure that your
+    documentation changes are clean.