add basic metadata for F-Droid.

Fixes https://github.com/termux/termux-app/issues/622.

fastlane/metadata/android/en-US/full_description.txt

@@ -0,0 +1,13 @@
+Termux is a terminal emulator application enhanced with a large set of command line utilities ported to Android OS. The main goal is to bring a Linux command line experience to users of mobile devices with no rooting or other special setup required.
+
+* Enjoy the Bash and Zsh shells.
+* Edit files with nano and vim.
+* Access servers over SSH.
+* Compile C/C++ code with clang.
+* Use the Python console as a pocket calculator.
+* Check out projects with Git and Subversion.
+* Run text-based games with frotz.
+
+At first start a small base system is being configured. The GNU Bash, Coreutils, Findutils and other core utilities are available out-of-box. Additionally, we provide more than 1000 other packages installable by using the 'pkg' utility which currently is a frontend for the 'apt' package manager. All provided software has been patched and compiled with Android NDK to provide max compatibility with Android OS.
+
+To learn more about application usage tips and tricks, long-press anywhere on the terminal and select the Help menu option to access Termux Wiki. This resource is also accessible directly in a web browser: https://wiki.termux.com/wiki/Main_Page.

fastlane/metadata/android/en-US/short_description.txt

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+Terminal emulator app with a large set of command line utilities