Rename master branch to main

[closes pgjdbc#290]

Author: mp911de

.github/CONTRIBUTING.adoc

@@ -14,7 +14,7 @@ First off, thank you for taking the time to contribute! 👍 🎉
 [[code-of-conduct]]
 == Code of Conduct
 
-This project is governed by the link:https://github.com/pgjdbc/r2dbc-postgresql/blob/master/.github/CODE_OF_CONDUCT.adoc[Code of Conduct].
+This project is governed by the link:https://github.com/pgjdbc/r2dbc-postgresql/blob/main/.github/CODE_OF_CONDUCT.adoc[Code of Conduct].
 By participating you are expected to uphold this code.
 Please report unacceptable behavior to [email protected].
 
@@ -63,7 +63,7 @@ The purpose of the ticket is to understand and discuss the underlying issue or f
 We use the JIRA issue tracker as the preferred place of record for conversations and conclusions.
 In that sense discussions directly under a PR are more implementation detail oriented and transient in nature.
 
-2. Always check out the `master` branch and submit pull requests against it.
+2. Always check out the `main` branch and submit pull requests against it.
 Backports to prior versions will be considered on a case-by-case basis and reflected as the fix version in the issue tracker.
 
 3. Use short branch names, preferably based on the GitHub issue (e.g. `gh-1234`), or otherwise using succinct, lower-case, dash (-) delimited names, such as `fix-warnings`.
@@ -87,5 +87,5 @@ In other words, you do not need to create a new pull request when asked to make
 [[source-code-style]]
 == Source Code Style
 
-We provide an IntelliJ link:https://github.com/pgjdbc/r2dbc-postgresql/blob/master/intellij-style.xml[IDEA code formatting configuration] that defines the source file coding standards.
+We provide an IntelliJ link:https://github.com/pgjdbc/r2dbc-postgresql/blob/main/intellij-style.xml[IDEA code formatting configuration] that defines the source file coding standards.
 Import and use the provided configuration to avoid formatting changes in pull requests.

.github/pull_request_template.md

@@ -2,7 +2,7 @@
 
 Make sure that:
 
-- [ ] You have read the [contribution guidelines](https://github.com/pgjdbc/r2dbc-postgresql/blob/master/.github/CONTRIBUTING.adoc).
+- [ ] You have read the [contribution guidelines](https://github.com/pgjdbc/r2dbc-postgresql/blob/main/.github/CONTRIBUTING.adoc).
 - [ ] You have created a feature request first to discuss your contribution intent. Please reference the feature request ticket number in the pull request.
 - [ ] You use the code formatters provided [here](https://github.com/pgjdbc/r2dbc-postgresql/blob/master/intellij-style.xml) and have them applied to your changes. Don't submit any formatting related changes.
 - [ ] You submit test cases (unit or integration tests) that back your changes.

README.md

@@ -1,4 +1,4 @@
-# PostgreSQL R2DBC Driver [![Build Status](https://travis-ci.org/r2dbc/r2dbc-postgresql.svg?branch=master)](https://travis-ci.org/r2dbc/r2dbc-postgresql) [![Maven Central](https://maven-badges.herokuapp.com/maven-central/io.r2dbc/r2dbc-postgresql/badge.svg)](https://maven-badges.herokuapp.com/maven-central/io.r2dbc/r2dbc-postgresql)
+# PostgreSQL R2DBC Driver [![Build Status](https://travis-ci.org/r2dbc/r2dbc-postgresql.svg?branch=main)](https://travis-ci.org/r2dbc/r2dbc-postgresql) [![Maven Central](https://maven-badges.herokuapp.com/maven-central/io.r2dbc/r2dbc-postgresql/badge.svg)](https://maven-badges.herokuapp.com/maven-central/io.r2dbc/r2dbc-postgresql)
 
 This project contains the [PostgreSQL][p] implementation of the [R2DBC SPI][r].  This implementation is not intended to be used directly, but rather to be used as the backing implementation for a humane client library to delegate to.