[doc] Document the renaming from master to main (#116)

* [doc] Document the renaming from master to main

The default branch is now `main` instead of `master`.

The `master` branch is not updated anymore, and therefore
can generate some warning messages.

This commit adds instructions to explain how to update
the local configuration to point to the new `main` branch.

Relates #115 #110

README.md

@@ -2,6 +2,15 @@
 
 This tap is for products in the Elastic stack.
 
+**WARNING: main is now the default branch**
+
+Since October 2021, the default branch is renamed from `master` to `main`.
+
+The `master` branch is no longer updated and will be **deleted on June 1st, 2023**.
+
+Please [follow the instructions to update](#how-do-i-ensure-my-configuration-is-up-to-date) your configuration.
+
+
 ## How do I install these formulae?
 
 Install the tap via:
@@ -31,5 +40,30 @@ too; replace `-full` with `-oss` in any of the above commands to install the
 OSS distribution. Note that the default distribution and OSS distribution of
 a product can not be installed at the same time.
 
+## How do I ensure my configuration is still up to date after the branch renaming?
+
+Run the following command to update your configuration:
+
+    brew untap elastic/tap --force
+    brew tap elastic/tap
+
+Verify your configuration is based on the `main` branch with:
+
+    git -C /usr/local/homebrew/Library/Taps/elastic/homebrew-tap status
+
+You should have the following output:
+
+    On branch main
+    Your branch is up to date with 'origin/main'.
+
 ## Documentation
 `brew help`, `man brew` or check [Homebrew's documentation](https://github.com/Homebrew/brew/blob/master/docs/README.md).
+
+## Troubleshooting
+### Calling bottle :unneeded is deprecated!
+
+When I execute `brew update`, the following warning appears
+
+    Warning: Calling bottle :unneeded is deprecated! There is no replacement.
+
+This is related to your configuration not being up-to-date. Your are still using the legacy `master` branch which is not updated anymore. Please [follow the instructions to update](#how-do-i-ensure-my-configuration-is-up-to-date) your local configuration to use the `main` branch.