Add the BSD-4-Clause license

This adds the BSD-4-clause license type.

1. SPDX Identifier: https://spdx.org/licenses/BSD-4-Clause.html
2. GNU's list of free licenses: https://www.gnu.org/licenses/license-list.en.html#OriginalBSD and FSF approved
3. Over 1000 projects: https://github.com/search?q=BSD+4-Clause+filename%3ALICENSE&type=Code
4. Yosemite Blockchain, pwolfgang/querybuilder and PMSPAUR-public use the BSD-4-clause license

This is based on the previous work of: https://github.com/github/choosealicense.com/pull/511

Signed-off-by: Alistair Francis <alistair@alistair23.me>

_licenses/bsd-4-clause.txt

@@ -0,0 +1,62 @@
+---
+title: BSD 4-Clause "Original" or "Old" License
+spdx-id: BSD-4-Clause
+hidden: true
+
+description: A permissive license similar to the <a href="/licenses/bsd-3-clause/">BSD 3-Clause License</a>, but with a 4th clause (known as the "advertising clause") that requires an acknowledgment of the original source in all advertising material.
+
+how: Create a text file (typically named LICENSE or LICENSE.txt) in the root of your source code and copy the text of the license into the file. Replace [year] with the current year and [fullname] with the name (or names) of the copyright holders. Replace [project] with the project organization, if any, that sponsors this work.
+
+using:
+  - Yosemite Blockchain: https://github.com/YosemiteLabs/yosemite-public-blockchain/blob/master/LICENSE
+  - querybuilder: https://github.com/pwolfgang/querybuilder/blob/master/LICENSE
+  - PMSPAUR-public: https://github.com/ArthurGodet/PMSPAUR-public/blob/88b6d0fd172950a7e5e1cc595d180565cfaa5462/LICENSE
+
+permissions:
+  - commercial-use
+  - modifications
+  - distribution
+  - private-use
+
+conditions:
+  - include-copyright
+
+limitations:
+  - liability
+  - warranty
+
+---
+
+BSD 4-Clause License
+
+Copyright (c) [year], [fullname]
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+1. Redistributions of source code must retain the above copyright notice, this
+   list of conditions and the following disclaimer.
+
+2. Redistributions in binary form must reproduce the above copyright notice,
+   this list of conditions and the following disclaimer in the documentation
+   and/or other materials provided with the distribution.
+
+3. All advertising materials mentioning features or use of this software must
+   display the following acknowledgement:
+     This product includes software developed by [project].
+
+4. Neither the name of the copyright holder nor the names of its
+   contributors may be used to endorse or promote products derived from
+   this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY COPYRIGHT HOLDER "AS IS" AND ANY EXPRESS OR
+IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO
+EVENT SHALL COPYRIGHT HOLDER BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
+OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
+OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.