[AIRFLOW-1603] add PAYMILL to companies list
[AIRFLOW-1603] add PAYMILL to companies list

Dear Airflow maintainers,

Please accept this PR. I understand that it will
not be reviewed until I have checked off all the
steps below!

- [x] My PR addresses the following [Airflow JIRA]
(https://issues.apache.org/jira/browse/AIRFLOW/)
issues and references them in the PR title. For
example, "[AIRFLOW-XXX] My Airflow PR"
    -
https://issues.apache.org/jira/browse/AIRFLOW-1603

- [x] Here are some details about my PR, including
screenshots of any UI changes:
  - added my company to README.md

- [x] My PR adds the following unit tests __OR__
does not need testing for this extremely good
reason:

- [x] My commits all reference JIRA issues in
their subject lines, and I have squashed multiple
commits if they address the same issue. In
addition, my commits follow the guidelines from
"[How to write a good git commit
message](http://chris.beams.io/posts/git-
commit/)":
    1. Subject is separated from body by a blank line
    2. Subject is limited to 50 characters
    3. Subject does not end with a period
    4. Subject uses the imperative mood ("add", not
"adding")
    5. Body wraps at 72 characters
    6. Body explains "what" and "why", not "how"

Closes #2603 from matthiashuschle/pr2

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low,sekikn/incubator-airflow,jfantom/incubator-airflow,gtoonstra/airflow,apache/airflow,sid88in/incubator-airflow,mistercrunch/airflow,mrares/incubator-airflow,owlabs/incubator-airflow,spektom/incubator-airflow,skudriashev/incubator-airflow,bolkedebruin/airflow,malmiron/incubator-airflow,airbnb/airflow,jfantom/incubator-airflow,nathanielvarona/airflow,yati-sagade/incubator-airflow,danielvdende/incubator-airflow,KL-WLCR/incubator-airflow