Update the basic documentation.

INSTALL.md

@@ -0,0 +1,22 @@
+# How to compile and install mboxgrep
+
+mboxgrep should compile on a modern Unix-like operating system, such as GNU/Linux or FreeBSD.
+
+It uses autoconf, so the most basic compilation procedure consists of:
+
+```
+./configure
+make
+make install # root rights probably needed here, prefix with sudo in such case
+```
+
+To see the list of flags accepted by the configure script, run:
+
+```
+./configure --help
+```
+
+Optionally, `mboxgrep` can be linked with the following libraries:
+
+- PCRE, to enable support for regular expressions compatible with Perl 5;
+- zlib and bzlib, to enable support for compressed mbox folders.

README.md

@@ -1,4 +1,7 @@
-mboxgrep - displays e-mail messages matching a pattern
+# mboxgrep - selects e-mail messages matching a pattern
+
+`mboxgrep` is a `grep(1)`-like tool which scans mailboxes and selects
+e-mail messages matching pattern.
 
 Full description of mboxgrep is contained in the documentation,
 which is provided both in manpage and texinfo format, to satisfy
@@ -11,8 +14,8 @@ are welcome.
 If you intend to bundle mboxgrep with an operating system (such as a
 GNU/Linux distrubution, for example), please drop me a line about it.
 
-For build instructions, read the file "INSTALL", and run
+For build instructions, read the file `INSTALL.md`, and run
 `./configure --help`.
 
-This project aims to follow the GNU coding style (https://www.gnu.org/prep/standards/html_node/Formatting.html),
-at least loosely. The code should be indented with indent -gnu -nut -ppi2.
+This project aims to follow the [GNU coding style](https://www.gnu.org/prep/standards/html_node/Formatting.html),
+at least loosely. The code should be indented with `indent -gnu -nut -ppi2`.