pushできていたリポジトリにpushできなくなった。
恐らくREADME.mdの内容をgithubのサイト上で変更したことが原因だと思われるが、解決方法がわからない。

結論から言えば、この記事通りにやったら無事解決できた。

以下、一通り出たエラーとやったことを書き残しておく。

$git push origin master 
 ! [rejected]        master -> master (fetch first)
error: failed to push some refs to 'https://github.com/…/リポジトリ名.git'
hint: Updates were rejected because the remote contains work that you do
hint: not have locally. This is usually caused by another repository pushing
hint: to the same ref. You may want to first integrate the remote changes
hint: (e.g., 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

とりあえずpullしろと言われているようなので、pullしてみたが似たようなエラーが。

$ git pull origin master
remote: Enumerating objects: 5, done.
remote: Counting objects: 100% (5/5), done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 3 (delta 1), reused 0 (delta 0), pack-reused 0
Unpacking objects: 100% (3/3), done.
From https://github.com/ユーザー名/リポジトリ名
 * branch            master     -> FETCH_HEAD
   060ee7a..4c90029  master     -> origin/master
Auto-merging README.md
CONFLICT (content): Merge conflict in README.md
Automatic merge failed; fix conflicts and then commit the result.

やはりREADME.mdが原因の様子。
正直gitはpushとpullしかしたことがない私には、mergeがなんなのかよくわからないが、とりあえずエラーをググると$git pushで解決できると書いてあったので実行してみた。

$ git push
fatal: The current branch master has no upstream branch.
To push the current branch and set the remote as upstream, use

    git push --set-upstream origin master

$git push --set-upstream origin master
To https://github.com/ユーザー名/リポジトリ名.git
 ! [rejected]        master -> master (non-fast-forward)
error: failed to push some refs to 'https://github.com/ユーザー名/リポジトリ名.git'
hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart. Integrate the remote changes (e.g.
hint: 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

うーん。やってみたが解決しないので、もう一度git push origin masterしてみる

$ git push origin master
To https://github.com/ユーザー名/リポジトリ名.git
 ! [rejected]        master -> master (non-fast-forward)
error: failed to push some refs to 'https://github.com/ユーザー名/リポジトリ名.git'
hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart. Integrate the remote changes (e.g.
hint: 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.
$ git pull origin master
error: Pulling is not possible because you have unmerged files.
hint: Fix them up in the work tree, and then use 'git add/rm <file>'
hint: as appropriate to mark resolution and make a commit.
fatal: Exiting because of an unresolved conflict.

と、若干違うエラーが出たので、再度検索してみたら出てきたこの記事を参考に、その通りに実行したら無事解決！

0$ git add -A
$ git commit
[master 015118d] Merge branch 'master' of https://github.com/ユーザー名/リポジトリ名

$ git pull origin master
From https://github.com/ユーザー名/リポジトリ名
 * branch            master     -> FETCH_HEAD
Already up to date.

$ git push origin master
Enumerating objects: 52, done.
Counting objects: 100% (38/38), done.
Delta compression using up to 4 threads
Compressing objects: 100% (20/20), done.
Writing objects: 100% (20/20), 2.46 KiB | 838.00 KiB/s, done.
Total 20 (delta 10), reused 0 (delta 0)
remote: Resolving deltas: 100% (10/10), completed with 8 local objects.
remote:
remote: GitHub found 5 vulnerabilities on ユーザー名/リポジトリ名's default branch (1 high, 3 moderate, 1 low). To find out more, visit:
remote:      https://github.com/ユーザー名/リポジトリ名/network/alerts
remote:
To https://github.com/ユーザー名/リポジトリ名.git
   4c90029..015118d  master -> master