!5 Add README.md.

From: @xielhxie
Reviewed-by: @hanxinke
Signed-off-by: @hanxinke

README.md

@@ -0,0 +1,49 @@
+# sed
+
+sed is a stream editor.
+sed is a non-interactive command-line text editor.
+A stream editor is used to perform basic text transformations on an input stream (a file or input from a pipeline). 
+While in some ways similar to an editor which permits scripted edits (such as ed), sed works by making only one pass over the input(s), and is consequently more efficient. 
+But it is sed’s ability to filter text in a pipeline which particularly distinguishes it from other types of editors.
+
+Example: delete the 4th line in a file
+$ sed '4d' input.txt > output.txt
+
+Example: replace every occurrence of 'hello' with 'world' on lines 10-20
+$ sed '10,20s/hello/world/' input.txt > output.txt
+sed is commonly used to filter text, i.e., it takes text input, performs some operation (or set of operations) on it, and outputs the modified text. 
+sed is typically used for extracting part of a file using pattern matching or substituting multiple occurrences of a string within a file.
+
+### Overview
+Normally sed is invoked like this:
+
+sed SCRIPT INPUTFILE...
+For example, to replace all occurrences of ‘hello’ to ‘world’ in the file input.txt:
+
+sed 's/hello/world/' input.txt > output.txt
+If you do not specify INPUTFILE, or if INPUTFILE is -, sed filters the contents of the standard input. The following commands are equivalent:
+
+sed 's/hello/world/' input.txt > output.txt
+sed 's/hello/world/' < input.txt > output.txt
+cat input.txt | sed 's/hello/world/' - > output.txt
+sed writes output to standard output. Use -i to edit files in-place instead of printing to standard output. See also the W and s///w commands for writing output to other files. The following command modifies file.txt and does not produce any output:
+
+sed -i 's/hello/world/' file.txt
+By default sed prints all processed input (except input that has been modified/deleted by commands such as d). Use -n to suppress output, and the p command to print specific lines. The following command prints only line 45 of the input file:
+
+sed -n '45p' file.txt
+sed treats multiple input files as one long stream. The following example prints the first line of the first file (one.txt) and the last line of the last file (three.txt). Use -s to reverse this behavior.
+
+sed -n  '1p ; $p' one.txt two.txt three.txt
+Without -e or -f options, sed uses the first non-option parameter as the script, and the following non-option parameters as input files. If -e or -f options are used to specify a script, all non-option parameters are taken as input files. Options -e and -f can be combined, and can appear multiple times (in which case the final effective script will be concatenation of all the individual scripts).
+
+The following examples are equivalent:
+
+sed 's/hello/world/' input.txt > output.txt
+
+sed -e 's/hello/world/' input.txt > output.txt
+sed --expression='s/hello/world/' input.txt > output.txt
+
+echo 's/hello/world/' > myscript.sed
+sed -f myscript.sed input.txt > output.txt
+sed --file=myscript.sed input.txt > output.txt