Sitecore 8.1 custom index configuration example for web db. To be placed in \app_config\include\customer-folder. Depends on a custom BaseConfiguration file

ContentSearch.CustomIndex.config

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>
    <contentSearch>
      <configuration type="Sitecore.ContentSearch.ContentSearchConfiguration, Sitecore.ContentSearch">
        <indexes hint="list:AddIndex">
          <index id="yourcustom_indexname" type="Sitecore.ContentSearch.LuceneProvider.LuceneIndex, Sitecore.ContentSearch.LuceneProvider">
            <param desc="name">$(id)</param>
            <param desc="folder">$(id)</param>
            <!-- This initializes index property store. Id has to be set to the index id -->
            <param desc="propertyStore" ref="contentSearch/indexConfigurations/databasePropertyStore" param1="$(id)" />
            <configuration ref="contentSearch/indexConfigurations/CustomIndexBaseConfiguration" />
            <strategies hint="list:AddStrategy">
              <!-- NOTE: order of these is controls the execution order -->
              <strategy ref="contentSearch/indexConfigurations/indexUpdateStrategies/onPublishEndAsync" />
            </strategies>
            <commitPolicyExecutor type="Sitecore.ContentSearch.CommitPolicyExecutor, Sitecore.ContentSearch">
              <policies hint="list:AddCommitPolicy">
                <policy type="Sitecore.ContentSearch.ModificationCountCommitPolicy, Sitecore.ContentSearch">
                  <Limit>300</Limit>
                </policy>
              </policies>
            </commitPolicyExecutor>
            <locations hint="list:AddCrawler">
              <crawler type="Sitecore.ContentSearch.SitecoreItemCrawler, Sitecore.ContentSearch">
                <Database>web</Database>
                <Root>/sitecore/content/Your-website-root</Root>
              </crawler>
            </locations>
            <enableItemLanguageFallback>false</enableItemLanguageFallback>
            <enableFieldLanguageFallback>false</enableFieldLanguageFallback>
            <!--
            <shardingStrategy type="Sitecore.ContentSearch.LuceneProvider.Sharding.LucenePartitionShardingStrategy, Sitecore.ContentSearch.LuceneProvider">
              <param desc="shardDistribution">4</param>
            </shardingStrategy>
            -->

            <!--
            <shardFolders hint="raw:AddShardFolderPath">
              <shard shardName="shard1" shardRootFolderPath="c:\Data\Indexes" />
              <shard shardName="shard2" shardRootFolderPath="c:\Data\Indexes" />
              <shard shardName="shard3" shardRootFolderPath="c:\Data\Indexes" />
            </shardFolders>
            -->
          </index>
        </indexes>
      </configuration>
    </contentSearch>
  </sitecore>
</configuration>