Documents JavaScript runtime requirement

Related issue:

$ jekyll serve
/usr/local/share/gems/gems/execjs-2.2.1/lib/execjs/runtimes.rb:51:in
`autodetect': Could not find a JavaScript runtime. See
https://github.com/sstephenson/execjs for a list of available runtimes.
(ExecJS::RuntimeUnavailable)
Documents Jekyll-Sitemap requirement

Related issue:

$ jekyll serve
Configuration file:
/home/dhiana/Desktop/CS/concretesolutions.github.io/_config.yml
/usr/share/rubygems/rubygems/core_ext/kernel_require.rb:55:in `require':
cannot load such file -- jekyll-sitemap (LoadError)
	from
/usr/share/rubygems/rubygems/core_ext/kernel_require.rb:55:in `require'
	from
/usr/local/share/gems/gems/jekyll-2.3.0/lib/jekyll/plugin_manager.rb:28:in
`block in require_gems'
	from
/usr/local/share/gems/gems/jekyll-2.3.0/lib/jekyll/plugin_manager.rb:26:in
`each'
	from
/usr/local/share/gems/gems/jekyll-2.3.0/lib/jekyll/plugin_manager.rb:26:in
`require_gems'
	from
/usr/local/share/gems/gems/jekyll-2.3.0/lib/jekyll/plugin_manager.rb:19:in
`conscientious_require'
	from
/usr/local/share/gems/gems/jekyll-2.3.0/lib/jekyll/site.rb:73:in `setup'
	from
/usr/local/share/gems/gems/jekyll-2.3.0/lib/jekyll/site.rb:35:in
`initialize'
	from
/usr/local/share/gems/gems/jekyll-2.3.0/lib/jekyll/commands/build.rb:28:in
`new'
	from
/usr/local/share/gems/gems/jekyll-2.3.0/lib/jekyll/commands/build.rb:28:in
`process'
	from
/usr/local/share/gems/gems/jekyll-2.3.0/lib/jekyll/commands/serve.rb:24:in
`block (2 levels) in init_with_program'
	from
/usr/local/share/gems/gems/mercenary-0.3.4/lib/mercenary/command.rb:220:in
`call'
	from
/usr/local/share/gems/gems/mercenary-0.3.4/lib/mercenary/command.rb:220:in
`block in execute'
	from
/usr/local/share/gems/gems/mercenary-0.3.4/lib/mercenary/command.rb:220:in
`each'
	from
/usr/local/share/gems/gems/mercenary-0.3.4/lib/mercenary/command.rb:220:in
`execute'
	from
/usr/local/share/gems/gems/mercenary-0.3.4/lib/mercenary/program.rb:35:in
`go'
	from
/usr/local/share/gems/gems/mercenary-0.3.4/lib/mercenary.rb:22:in
`program'
	from /usr/local/share/gems/gems/jekyll-2.3.0/bin/jekyll:18:in
`<top (required)>'
	from /usr/local/bin/jekyll:23:in `load'
	from /usr/local/bin/jekyll:23:in `<main>'
Adds temp swp files to gitignore