filter-repo (README): add code of conduct and contributing guidelines

Signed-off-by: Elijah Newren <newren@gmail.com>

Documentation/Contributing.md

@@ -0,0 +1,57 @@
+Welcome to the community!  Contributions to filter-repo generally
+should follow the same [guidelines for contribution to
+git.git](https://git.kernel.org/pub/scm/git/git.git/tree/Documentation/SubmittingPatches).
+
+Exceptions:
+
+  * While I
+    [hate](https://public-inbox.org/git/CABPp-BG2SkH0GrRYpHLfp2Wey91ThwQoTgf9UmPa9f5Szn+v3Q@mail.gmail.com/)
+    [GitHub](https://public-inbox.org/git/CABPp-BEcpasV4vBTm0uxQ4Vzm88MQAX-ArDG4e9QU8tEoNsZWw@mail.gmail.com/)
+    [PRs](https://public-inbox.org/git/CABPp-BEHy8c3raHwf9aFXvXN0smf_WwCcNiYxQBwh7W6An60qQ@mail.gmail.com/),
+    git-format-patch and git-send-email can be a beast and I have not
+    yet found time to modify Dscho's excellent
+    [GitGitGadget](https://github.com/gitgitgadget/gitgitgadget) to
+    work with git-filter-repo.  As such:
+      * For very short single-commit changes, feel free to open GitHub PRs.
+      * For more involved changes, if format-patch or send-email give you
+        too much trouble, go ahead and open a GitHub PR and just mention
+        that email didn't work out.
+  * When emailing the git list:
+    * Include "filter-repo" at the start of the subject,
+      e.g. "[filter-repo PATCH] Add packaging scripts for uploading to PyPI"
+      instead of just "[PATCH] Add packaging scripts for uploading to PyPI"
+    * CC me instead of the git maintainer
+  * Git's [CodingGuidlines for python
+    code](https://github.com/git/git/blob/v2.24.0/Documentation/CodingGuidelines#L482-L494)
+    are only partially applicable:
+    * python3 is a hard requirement; python2 is/was EOL at the end of
+      2019 and should not be used.  (Commit 4d0264ab723c
+      ("filter-repo: workaround python<2.7.9 exec bug", 2019-04-30)
+      was the last version of filter-repo that worked with python2).
+    * You can depend on anything in python 3.5 or earlier.  I may bump
+      this minimum version over time, but do want to generally work
+      with the python3 version found in current enterprise Linux
+      distributions.
+    * In filter-repo, it's not just OK to use bytestrings, you are
+      expected to use them a lot.  Using unicode strings result in
+      lots of ugly errors since input comes from filesystem names,
+      commit messages, file contents, etc., none of which are
+      guaranteed to be unicode.  (Plus unicode strings require lots of
+      effort to verify, encode, and decode -- slowing the filtering
+      process down).  I tried to work with unicode strings more
+      broadly in the code base multiple times; but it's just a bad
+      idea to use an abstraction that doesn't fit the data.
+    * I generally like [PEP
+      8](https://www.python.org/dev/peps/pep-0008/), but have used
+      two-space indents for years anyway -- and expect contributions
+      to do the same.
+
+Also, please do not be intimidated by detailed feedback:
+
+ * In the git community, I have been contributing for years and have
+   had hundreds of patches accepted but I still find that even when I
+   try to make patches perfect I am not surprised when I have to spend
+   as much or more time fixing up patches after submitting them than I
+   did figuring out the patches in the first place.  git folks tend to
+   do thorough reviews, which has taught me a lot, and I try to do the
+   same for filter-repo.

README.md

@@ -334,6 +334,17 @@ one of the last four traits as well:
 
   1. [Speed] Filtering should be reasonably fast
 
+# How do I contribute?
+
+See the [contributing guidelines](Documentation/Contributing.md).
+
+# Is there a Code of Conduct?
+
+Participants in the filter-repo community are expected to adhere to
+the same standards as for the git project, so the [git Code of
+Conduct](https://git.kernel.org/pub/scm/git/git.git/tree/CODE_OF_CONDUCT.md)
+applies.
+
 # Upstream Improvements
 
 Work on filter-repo and [its