lumps: Add README files for some directories.

These deserve to be documented.

lumps/colormap/README

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+This directory contains the generator script for the COLORMAP lump. The
+COLORMAP lump is used to calculate Doom's lighting; it translates normal
+colors into darkened versions in 32 different levels of light. Some more
+details can be found on the Doom wiki:
+
+   <http://doomwiki.org/wiki/COLORMAP>
+
+Boom extends the COLORMAP to allow the addition of extra colormap lumps;
+these can be used to apply colored lighting and other effects.
+

lumps/playpal/README

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+This directory contains the palette generation script. The output of the
+script is the PLAYPAL lump.
+
+The Doom engine operates in a palettized screen mode where only 256
+different colors can be shown on screen at a given time. By applying
+alternate palettes, special effects can be generated (eg. the red tint
+when taking damage, or the green tint when wearing the radsuit). More
+information can be found on the Doom wiki:
+
+    <http://doomwiki.org/wiki/PLAYPAL>
+
+The input for the script is playpal-base.lmp which contains the default
+palette seen during normal gameplay. The script translates this palette
+into the various other forms which are shown in different situations.
+

lumps/textures/README

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+This directory contains the texture lump generation script. This is used
+to generate the TEXTURE1, TEXTURE2 and PNAMES lumps. The Doom engine uses
+these lumps on startup to build the game's wall textures from individual
+patch textures. More information can be found on the Doom wiki:
+
+    <http://doomwiki.org/wiki/TEXTURE1_and_TEXTURE2>
+
+The deutex tool contains its own texture builder. However, due to the
+specific requirements of the Freedoom project, we have implemented our
+own texture lump builder. See the header comments in the build-textures
+script for more information.
+