Merge #14497: docs: Add doc/bitcoin-conf.md

1fb3c167c3cbd4a432a064b299439b3430157dda Add `doc/bitcoin-conf.md` (Hennadii Stepanov)

Pull request description:

  From the IRC:
  > 2018-10-16T05:35:03  \<wumpus\> if something can be solved by better documentation, please work on documentation!
  > 2018-10-16T05:35:12  \<wumpus\> don't change the code instead

  Refs:

  - #14370
  - #14427
  - #14494

  Based on the BITCOIN.CONF(5) manual page written by Micah Anderson \<micah@debian.org\> for the Debian system.

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+# `bitcoin.conf` Configuration File
+
+The configuration file is used by `bitcoind`, `bitcoin-qt` and `bitcoin-cli`.
+
+All command-line options (except for `-?`, `-help`, `-version` and `-conf`) may be specified in a configuration file, and all configuration file options (except for `includeconf`) may also be specified on the command line. Command-line options override values set in the configuration file and configuration file options override values set in the GUI.
+
+## Configuration File Format
+
+The configuration file is a plain text file and consists of `option=value` entries, one per line. Leading and trailing whitespaces are removed.
+
+In contrast to the command-line usage:
+- an option must be specified without leading `-`;
+- a value of the given option is mandatory; e.g., `testnet=1` (for chain selection options), `noconnect=1` (for negated options).
+
+### Blank lines
+
+Blank lines are allowed and ignored by the parser.
+
+### Comments
+
+A comment starts with a number sign (`#`) and extends to the end of the line. All comments are ignored by the parser.
+
+Comments may appear in two ways:
+- on their own on an otherwise empty line (_preferable_);
+- after an `option=value` entry.
+
+### Network specific options
+
+Network specific options can be:
+- placed into sections with headers `[main]` (not `[mainnet]`), `[test]` (not `[testnet]`) or `[regtest]`;
+- prefixed with a chain name; e.g., `regtest.maxmempool=100`.
+
+## Configuration File Path
+
+The configuration file is not automatically created; you can create it using your favorite text editor. By default, the configuration file name is `bitcoin.conf` and it is located in the Bitcoin data directory, but both the Bitcoin data directory and the configuration file path may be changed using the `-datadir` and `-conf` command-line options.
+
+The `includeconf=<file>` option in the `bitcoin.conf` file can be used to include additional configuration files.