Clarify licensing

htop has been licensed as GPLv2 but there was inconsistency regarding the
option to choose "any later version" as granted by the license.

This commit clarifies the htop dev team is fine with that choice.

README

@@ -6,7 +6,7 @@
 [![IRC #htop](https://img.shields.io/badge/IRC-htop-blue.svg)](https://web.libera.chat/#htop)
 [![GitHub Release](https://img.shields.io/github/release/htop-dev/htop.svg)](https://github.com/htop-dev/htop/releases/latest)
 [![Packaging status](https://repology.org/badge/tiny-repos/htop.svg)](https://repology.org/project/htop/versions)
-[![License: GPL v2](https://img.shields.io/badge/License-GPL%20v2-blue.svg)](COPYING?raw=true)
+[![License: GPL v2+](https://img.shields.io/badge/License-GPL%20v2-blue.svg)](COPYING?raw=true)
 
 ![Screenshot of htop](docs/images/screenshot.png?raw=true)
 
@@ -174,4 +174,4 @@ In 2020 a [team](https://github.com/orgs/htop-dev/people) took over the developm
 
 ## License
 
-GNU General Public License, version 2 (GPL-2.0)
+GNU General Public License, version 2 (GPL-2.0) or, at your option, any later version.

docs/styleguide.md

@@ -45,8 +45,8 @@ Example:
 #define HEADER_FILENAME
 /*
 htop - Filename.h
-(C) 2020 htop dev team
-Released under the GNU GPLv2, see the COPYING file
+(C) 2021 htop dev team
+Released under the GNU GPLv2+, see the COPYING file
 in the source distribution for its full text.
 */
 ```
@@ -54,6 +54,8 @@ in the source distribution for its full text.
 Import and use of headers
 -------------------------
 
+We use the GPLv2+ as a shorthand indication that we release `htop` under the GNU Public license version 2 but are totally fine with users opting to apply the "any later version" clause.
+
 Every file should import headers for all symbols it's using.
 Thus when using a symbol from a header, even if that symbol is already imported by something else you use, you should declare an import for that header.
 Doing so allows for easier restructuring of the code when things need to be moved around.