log-snippet: add README

log-snippet/README.md

+# log-snippet
+
+log-snippet takes an arbitrary "compilation-log" and shows contextual lines, a bit like `grep -C`
+
+# Synopsis
+
+	log-snippet [-C CONTEXTLINES] < LOG
+
+# Example
+
+Take for example the output of `flake8` command:
+
+```
+% flake8 bad.py other.py 
+bad.py:4:8: E225 missing whitespace around operator
+bad.py:17:5: E303 too many blank lines (2)
+other.py:3:10: E201 whitespace after '('
+```
+
+... which yields filenames and line numbers, one per line.
+
+For each file/line number, `log-snippet` will show the content of the line number of the file, with optional context lines.
+
+For example, pipe `flake8` output into `log-snippet` and show up to 2 lines before and 2 lines after around the mentioned line:
+
+```
+% flake8 bad.py other.py | ~/progextern/attic/log-snippet/log-snippet.sh -C 2
+bad.py-2-
+bad.py-3-def foo1():
+bad.py:4:    qux=1
+bad.py-5-    print(qux)
+bad.py-6-
+--
+bad.py-15-
+bad.py-16-
+bad.py:17:    c = 3
+bad.py-18-    print(a, b, c)
+--
+other.py-1-
+other.py-2-
+other.py:3:def foo2( ):
+other.py-4-    if(1):
+other.py-5-        print('foo')
+```
+
+The default is `-C 5`.
+
+# Compilation log format
+
+The format accepted by log-snippet is used by many compilers and linters, for example flake8, gcc, etc.
+
+`log-snippet` output format is similar to the format returned by `grep -H -n -C 5`.