Add README explaining how to use this fork

README.md

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+This repo is a fork of [bioconda/bioconda-recipes][1] for maintaining
+Nextstrain-related recipes and submitting PRs upstream.  It contains only
+ephemeral branches for use in PRs.
+
+## Set up
+
+First clone the `master` branch from the upstream repo:
+
+    git clone --origin upstream --single-branch --branch master https://github.com/bioconda/bioconda-recipes
+
+Then add this repo as an additional remote:
+
+    cd bioconda-recipes
+    git remote add --fetch nextstrain https://github.com/nextstrain/bioconda-recipes
+
+Now you have a local clone with two remotes, `upstream` and `nextstrain`,
+instead of the usual `origin`, and the `master` branch tracking
+`upstream/master`.
+
+## Starting a PR
+
+First make sure your `master` branch is up-to-date:
+
+    git switch master
+    git pull --ff-only
+
+Then make a new branch for your changes:
+
+    git switch --create some-branch-name
+
+[Do your work and make commits][2].  Optionally, [test your changes locally][3].
+
+Push your branch to the `nextstrain` remote:
+
+    git push -u nextstrain some-branch-name
+
+Finally, go to GitHub and [open a PR][4] for your branch.
+
+
+[1]: https://github.com/bioconda/bioconda-recipes
+[2]: https://bioconda.github.io/contributor/workflow.html#make-some-edits
+[3]: https://bioconda.github.io/contributor/building-locally.html
+[4]: https://bioconda.github.io/contributor/workflow.html#create-a-pull-request