Add README

ext/dba/README

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+These functions build the foundation for accessing Berkeley DB style 
+databases. 
+
+This is a general abstraction layer for several file-based databases. As
+such, functionality is limited to a common subset of features supported 
+by modern databases such as Sleepycat Software's DB2. (This is not to be 
+confused with IBM's DB2 software, which is supported through the ODBC 
+functions.) 
+
+This extensions allows to work with the following databases:
+dbm      DBM is the oldest (original) type of Berkeley DB style databases.
+         You should avoid it, if possible. We do not support the 
+         compatibility functions built into DB2 and gdbm, because they are 
+         only compatible on the source code level, but cannot handle the 
+         original dbm format.  
+ndbm     NDBM is a newer type and more flexible than dbm. It still has 
+         most of the arbitrary limits of dbm (therefore it is deprecated).  
+gdbm     GDBM is the GNU database manager.  
+db2      DB2 is Sleepycat Software's DB2. It's described as "a programmatic
+         toolkit that provides high-performance built-in database support 
+         for both standalone and client/server applications.  
+db3      DB3 is Sleepycat Software's DB3.  
+db4      DB4 is Sleepycat Software's DB4. This is available since PHP 5.0.
+cdb      CDB is "a fast, reliable, lightweight package for creating and 
+         reading constant databases." It is from the author of qmail and 
+         can be found at http://cr.yp.to/cdb.html. Since it is constant, 
+         we support only reading operations. And since PHP 4.3.0 we support 
+         writing (not updating) through the internal cdb library.  
+cdb_make Since PHP 4.3.0 we support creation (not updating) of cdb files 
+         when the bundled cdb library is used.  
+flatfile This is available since PHP 4.3.0 for compatibility with the 
+         deprecated dbm extension only and should be avoided. However you 
+         may use this where files were created in this format. That happens 
+         when configure could not find any external library.  
+inifile  This is available since PHP 4.3.3 to be able to modify php.ini 
+         files from within PHP scripts. When working with ini files you 
+         can pass arrays of the form array(0=>group,1=>value_name) or 
+         strings of the form "[group]value_name" where group is optional. 
+         As the functions dba_firstkey() and dba_nextkey() return string 
+         representations of the key there is a new function dba_key_split() 
+         available since PHP 5 which allows to convert the string keys into 
+         array keys without loosing FALSE.  
+qdbm     This is available since PHP 5.0.0. The qdbm library can be loaded 
+         from http://qdbm.sourceforge.net.  
+
+
+After configuring and compiling PHP you must execute the following test
+from commandline: 
+  php run-tests.php ext/dba. 
+This shows whether your combination of handlers works. Most problematic 
+are dbm and ndbm which conflict with many installations. The reason for 
+this is that on several systems these libraries are part of more than one
+other library. The configuration test only prevents you from configuring 
+malfaunctioning single handlers but not combinations.