Add PTB support and fallback for missing wget

* Add support for Discord PTB and update the documentation accordingly
* Add curl as a fallback when wget is not installed
* Fix installation directory being /opt/discord-stable regardless of version
* Remove suffix from the Stable version (now simply discord/Discord)
* Exit with exit code 1 on error
* Miscellaneous minor edits and fixes

README.md

@@ -6,16 +6,19 @@ Unofficial RPM package for Discord.
 ## How to use
 1. [Download](https://github.com/RPM-Outpost/discord/archive/master.zip) and extract the zip.
 2. Open a terminal and `cd` to the `discord-master` directory.
-3. Run `./create-package.sh stable` to get the stable version of Discord, or `./create-package.sh canary` to get the unstable beta version.
+3. Run one of the following depending on the version you want to create a package for:
+    - Discord Stable: `./create-package.sh stable`
+    - Discord PTB:     `./create-package.sh ptb`
+    - Discord Canary: `./create-package.sh canary`
 
 ## Features
 - Downloads the latest version of Discord from the official website
 - Creates a ready-to-use RPM package
-- Discord stable and canary can be installed at the same time
-- Adds Discord to the applications' list with a nice HD icon
+- Discord Stable, PTB, and Canary can be installed at the same time
+- Adds Discord to the applications list with a nice HD icon
 - Supports Fedora (26, 27), OpenSUSE (Leap) and CentOS (7.x)
 
-## More informations
+## More information
 
 ### Warning - no accents
 
@@ -23,9 +26,9 @@ The path where you run the script must **not** contain any special character lik
 
 ### How to update
 
-When a new version of discord is released, simply run the script again to get the updated version.
+When a new version of Discord is released, simply run the script again to get the updated version.
 
-### Requirements
+### Dependencies
 The `rpmdevtools` package is required to build RPM packages. The script detects if it isn't installed and offers to install it.
 
 ### About root privileges